diff --git a/src/isomorphic/classic/element/ReactCurrentOwner.js b/src/isomorphic/classic/element/ReactCurrentOwner.js
index fe90c214ef553..720de0acc4806 100644
--- a/src/isomorphic/classic/element/ReactCurrentOwner.js
+++ b/src/isomorphic/classic/element/ReactCurrentOwner.js
@@ -10,7 +10,6 @@
 
 'use strict';
 
-import type {ReactInstance} from 'ReactInstanceType';
 import type {Fiber} from 'ReactFiber';
 
 /**
@@ -24,7 +23,7 @@ var ReactCurrentOwner = {
    * @internal
    * @type {ReactComponent}
    */
-  current: (null: null | ReactInstance | Fiber),
+  current: (null: null | Fiber),
 };
 
 module.exports = ReactCurrentOwner;
diff --git a/src/renderers/dom/fiber/ReactDOMFiberEntry.js b/src/renderers/dom/fiber/ReactDOMFiberEntry.js
index 7edda99d997f9..aca08cbb0d29e 100644
--- a/src/renderers/dom/fiber/ReactDOMFiberEntry.js
+++ b/src/renderers/dom/fiber/ReactDOMFiberEntry.js
@@ -229,7 +229,7 @@ var DOMRenderer = ReactFiberReconciler({
     if (__DEV__) {
       // TODO: take namespace into account when validating.
       const hostContextDev = ((hostContext: any): HostContextDev);
-      validateDOMNesting(type, null, null, hostContextDev.ancestorInfo);
+      validateDOMNesting(type, null, hostContextDev.ancestorInfo);
       if (
         typeof props.children === 'string' ||
         typeof props.children === 'number'
@@ -240,7 +240,7 @@ var DOMRenderer = ReactFiberReconciler({
           type,
           null,
         );
-        validateDOMNesting(null, string, null, ownAncestorInfo);
+        validateDOMNesting(null, string, ownAncestorInfo);
       }
       parentNamespace = hostContextDev.namespace;
     } else {
@@ -295,7 +295,7 @@ var DOMRenderer = ReactFiberReconciler({
           type,
           null,
         );
-        validateDOMNesting(null, string, null, ownAncestorInfo);
+        validateDOMNesting(null, string, ownAncestorInfo);
       }
     }
     return diffProperties(
@@ -362,7 +362,7 @@ var DOMRenderer = ReactFiberReconciler({
   ): TextInstance {
     if (__DEV__) {
       const hostContextDev = ((hostContext: any): HostContextDev);
-      validateDOMNesting(null, text, null, hostContextDev.ancestorInfo);
+      validateDOMNesting(null, text, hostContextDev.ancestorInfo);
     }
     var textNode: TextInstance = createTextNode(text, rootContainerInstance);
     precacheFiberNode(internalInstanceHandle, textNode);
diff --git a/src/renderers/dom/shared/ReactDOMComponentFlags.js b/src/renderers/dom/shared/ReactDOMComponentFlags.js
deleted file mode 100644
index bc40a5a560f28..0000000000000
--- a/src/renderers/dom/shared/ReactDOMComponentFlags.js
+++ /dev/null
@@ -1,16 +0,0 @@
-/**
- * Copyright (c) 2015-present, Facebook, Inc.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- *
- * @providesModule ReactDOMComponentFlags
- */
-
-'use strict';
-
-var ReactDOMComponentFlags = {
-  hasCachedChildNodes: 1 << 0,
-};
-
-module.exports = ReactDOMComponentFlags;
diff --git a/src/renderers/dom/shared/ReactDOMComponentTree.js b/src/renderers/dom/shared/ReactDOMComponentTree.js
index abcb36b31f50f..e8969c183a14a 100644
--- a/src/renderers/dom/shared/ReactDOMComponentTree.js
+++ b/src/renderers/dom/shared/ReactDOMComponentTree.js
@@ -9,116 +9,18 @@
 
 'use strict';
 
-var DOMProperty = require('DOMProperty');
-var ReactDOMComponentFlags = require('ReactDOMComponentFlags');
 var {HostComponent, HostText} = require('ReactTypeOfWork');
-var {ELEMENT_NODE, COMMENT_NODE} = require('HTMLNodeType');
 
 var invariant = require('fbjs/lib/invariant');
 
-var ATTR_NAME = DOMProperty.ID_ATTRIBUTE_NAME;
-var Flags = ReactDOMComponentFlags;
-
 var randomKey = Math.random().toString(36).slice(2);
-
 var internalInstanceKey = '__reactInternalInstance$' + randomKey;
-
 var internalEventHandlersKey = '__reactEventHandlers$' + randomKey;
 
-/**
- * Check if a given node should be cached.
- */
-function shouldPrecacheNode(node, nodeID) {
-  return (
-    (node.nodeType === ELEMENT_NODE &&
-      node.getAttribute(ATTR_NAME) === '' + nodeID) ||
-    (node.nodeType === COMMENT_NODE &&
-      node.nodeValue === ' react-text: ' + nodeID + ' ') ||
-    (node.nodeType === COMMENT_NODE &&
-      node.nodeValue === ' react-empty: ' + nodeID + ' ')
-  );
-}
-
-/**
- * Drill down (through composites and empty components) until we get a host or
- * host text component.
- *
- * This is pretty polymorphic but unavoidable with the current structure we have
- * for `_renderedChildren`.
- */
-function getRenderedHostOrTextFromComponent(component) {
-  var rendered;
-  while ((rendered = component._renderedComponent)) {
-    component = rendered;
-  }
-  return component;
-}
-
-/**
- * Populate `_hostNode` on the rendered host/text component with the given
- * DOM node. The passed `inst` can be a composite.
- */
-function precacheNode(inst, node) {
-  var hostInst = getRenderedHostOrTextFromComponent(inst);
-  hostInst._hostNode = node;
-  node[internalInstanceKey] = hostInst;
-}
-
 function precacheFiberNode(hostInst, node) {
   node[internalInstanceKey] = hostInst;
 }
 
-function uncacheNode(inst) {
-  var node = inst._hostNode;
-  if (node) {
-    delete node[internalInstanceKey];
-    inst._hostNode = null;
-  }
-}
-
-/**
- * Populate `_hostNode` on each child of `inst`, assuming that the children
- * match up with the DOM (element) children of `node`.
- *
- * We cache entire levels at once to avoid an n^2 problem where we access the
- * children of a node sequentially and have to walk from the start to our target
- * node every time.
- *
- * Since we update `_renderedChildren` and the actual DOM at (slightly)
- * different times, we could race here and see a newer `_renderedChildren` than
- * the DOM nodes we see. To avoid this, ReactMultiChild calls
- * `prepareToManageChildren` before we change `_renderedChildren`, at which
- * time the container's child nodes are always cached (until it unmounts).
- */
-function precacheChildNodes(inst, node) {
-  if (inst._flags & Flags.hasCachedChildNodes) {
-    return;
-  }
-  var children = inst._renderedChildren;
-  var childNode = node.firstChild;
-  outer: for (var name in children) {
-    if (!children.hasOwnProperty(name)) {
-      continue;
-    }
-    var childInst = children[name];
-    var childID = getRenderedHostOrTextFromComponent(childInst)._domID;
-    if (childID === 0) {
-      // We're currently unmounting this child in ReactMultiChild; skip it.
-      continue;
-    }
-    // We assume the child nodes are in the same order as the child instances.
-    for (; childNode !== null; childNode = childNode.nextSibling) {
-      if (shouldPrecacheNode(childNode, childID)) {
-        precacheNode(childInst, childNode);
-        continue outer;
-      }
-    }
-    // We reached the end of the DOM children without finding an ID match.
-    invariant(false, 'Unable to find element with ID %s.', childID);
-  }
-  inst._flags |= Flags.hasCachedChildNodes;
-}
-
 /**
  * Given a DOM node, return the closest ReactDOMComponent or
  * ReactDOMTextComponent instance ancestor.
@@ -149,9 +51,6 @@ function getClosestInstanceFromNode(node) {
   }
   for (; node && (inst = node[internalInstanceKey]); node = parents.pop()) {
     closest = inst;
-    if (parents.length) {
-      precacheChildNodes(inst, node);
-    }
   }
 
   return closest;
@@ -166,18 +65,11 @@ function getInstanceFromNode(node) {
   if (inst) {
     if (inst.tag === HostComponent || inst.tag === HostText) {
       return inst;
-    } else if (inst._hostNode === node) {
-      return inst;
     } else {
       return null;
     }
   }
-  inst = getClosestInstanceFromNode(node);
-  if (inst != null && inst._hostNode === node) {
-    return inst;
-  } else {
-    return null;
-  }
+  return null;
 }
 
 /**
@@ -193,33 +85,7 @@ function getNodeFromInstance(inst) {
 
   // Without this first invariant, passing a non-DOM-component triggers the next
   // invariant for a missing parent, which is super confusing.
-  invariant(
-    inst._hostNode !== undefined,
-    'getNodeFromInstance: Invalid argument.',
-  );
-
-  if (inst._hostNode) {
-    return inst._hostNode;
-  }
-
-  // Walk up the tree until we find an ancestor whose DOM node we have cached.
-  var parents = [];
-  while (!inst._hostNode) {
-    parents.push(inst);
-    invariant(
-      inst._hostParent,
-      'React DOM tree root should always have a node reference.',
-    );
-    inst = inst._hostParent;
-  }
-
-  // Now parents contains each ancestor that does *not* have a cached native
-  // node, and `inst` is the deepest ancestor that does.
-  for (; parents.length; inst = parents.pop()) {
-    precacheChildNodes(inst, inst._hostNode);
-  }
-
-  return inst._hostNode;
+  invariant(false, 'getNodeFromInstance: Invalid argument.');
 }
 
 function getFiberCurrentPropsFromNode(node) {
@@ -234,9 +100,6 @@ var ReactDOMComponentTree = {
   getClosestInstanceFromNode: getClosestInstanceFromNode,
   getInstanceFromNode: getInstanceFromNode,
   getNodeFromInstance: getNodeFromInstance,
-  precacheChildNodes: precacheChildNodes,
-  precacheNode: precacheNode,
-  uncacheNode: uncacheNode,
   precacheFiberNode: precacheFiberNode,
   getFiberCurrentPropsFromNode,
   updateFiberProps,
diff --git a/src/renderers/dom/shared/ReactDOMEventListener.js b/src/renderers/dom/shared/ReactDOMEventListener.js
index 03f9bb1ddeafe..6950f06acae2a 100644
--- a/src/renderers/dom/shared/ReactDOMEventListener.js
+++ b/src/renderers/dom/shared/ReactDOMEventListener.js
@@ -31,22 +31,14 @@ function findRootContainerNode(inst) {
   // TODO: It may be a good idea to cache this to prevent unnecessary DOM
   // traversal, but caching is difficult to do correctly without using a
   // mutation observer to listen for all DOM changes.
-  if (typeof inst.tag === 'number') {
-    while (inst.return) {
-      inst = inst.return;
-    }
-    if (inst.tag !== HostRoot) {
-      // This can happen if we're in a detached tree.
-      return null;
-    }
-    return inst.stateNode.containerInfo;
-  } else {
-    while (inst._hostParent) {
-      inst = inst._hostParent;
-    }
-    var rootNode = ReactDOMComponentTree.getNodeFromInstance(inst);
-    return rootNode.parentNode;
+  while (inst.return) {
+    inst = inst.return;
+  }
+  if (inst.tag !== HostRoot) {
+    // This can happen if we're in a detached tree.
+    return null;
   }
+  return inst.stateNode.containerInfo;
 }
 
 // Used to store ancestor hierarchy in top level callback
diff --git a/src/renderers/dom/shared/ReactDOMFeatureFlags.js b/src/renderers/dom/shared/ReactDOMFeatureFlags.js
index 03ab7534ad331..43f4085015b2e 100644
--- a/src/renderers/dom/shared/ReactDOMFeatureFlags.js
+++ b/src/renderers/dom/shared/ReactDOMFeatureFlags.js
@@ -11,7 +11,6 @@
 
 var ReactDOMFeatureFlags = {
   fiberAsyncScheduling: false,
-  useFiber: true,
 };
 
 module.exports = ReactDOMFeatureFlags;
diff --git a/src/renderers/dom/shared/__tests__/ReactDOMComponentTree-test.js b/src/renderers/dom/shared/__tests__/ReactDOMComponentTree-test.js
index 147b6ba9ac84b..c3cf73bd457a9 100644
--- a/src/renderers/dom/shared/__tests__/ReactDOMComponentTree-test.js
+++ b/src/renderers/dom/shared/__tests__/ReactDOMComponentTree-test.js
@@ -23,17 +23,11 @@ describe('ReactDOMComponentTree', () => {
   }
 
   function getTypeOf(instance) {
-    if (typeof instance.tag === 'number') {
-      return instance.type;
-    }
-    return instance._currentElement.type;
+    return instance.type;
   }
 
   function getTextOf(instance) {
-    if (typeof instance.tag === 'number') {
-      return instance.memoizedProps;
-    }
-    return instance._stringText;
+    return instance.memoizedProps;
   }
 
   beforeEach(() => {
diff --git a/src/renderers/dom/shared/eventPlugins/SimpleEventPlugin.js b/src/renderers/dom/shared/eventPlugins/SimpleEventPlugin.js
index 8c53cf882b3d3..d33e45b936ba9 100644
--- a/src/renderers/dom/shared/eventPlugins/SimpleEventPlugin.js
+++ b/src/renderers/dom/shared/eventPlugins/SimpleEventPlugin.js
@@ -34,7 +34,7 @@ import type {
   DispatchConfig,
   ReactSyntheticEvent,
 } from 'ReactSyntheticEventType';
-import type {ReactInstance} from 'ReactInstanceType';
+import type {Fiber} from 'ReactFiber';
 import type {EventTypes, PluginModule} from 'PluginModuleType';
 
 /**
@@ -179,7 +179,7 @@ var SimpleEventPlugin: PluginModule<MouseEvent> = {
 
   extractEvents: function(
     topLevelType: TopLevelTypes,
-    targetInst: ReactInstance,
+    targetInst: Fiber,
     nativeEvent: MouseEvent,
     nativeEventTarget: EventTarget,
   ): null | ReactSyntheticEvent {
diff --git a/src/renderers/dom/shared/validateDOMNesting.js b/src/renderers/dom/shared/validateDOMNesting.js
index 7a153f264af94..ba6d55d813881 100644
--- a/src/renderers/dom/shared/validateDOMNesting.js
+++ b/src/renderers/dom/shared/validateDOMNesting.js
@@ -10,7 +10,6 @@
 'use strict';
 
 var emptyFunction = require('fbjs/lib/emptyFunction');
-var getComponentName = require('getComponentName');
 
 var validateDOMNesting = emptyFunction;
 
@@ -405,83 +404,9 @@ if (__DEV__) {
     return null;
   };
 
-  /**
-   * Given a ReactCompositeComponent instance, return a list of its recursive
-   * owners, starting at the root and ending with the instance itself.
-   */
-  var findOwnerStack = function(instance) {
-    if (!instance) {
-      return [];
-    }
-
-    var stack = [];
-    do {
-      stack.push(instance);
-    } while ((instance = instance._currentElement._owner));
-    stack.reverse();
-    return stack;
-  };
-
-  var getOwnerInfo = function(
-    childInstance,
-    childTag,
-    ancestorInstance,
-    ancestorTag,
-    isParent,
-  ) {
-    var childOwner = childInstance && childInstance._currentElement._owner;
-    var ancestorOwner =
-      ancestorInstance && ancestorInstance._currentElement._owner;
-
-    var childOwners = findOwnerStack(childOwner);
-    var ancestorOwners = findOwnerStack(ancestorOwner);
-
-    var minStackLen = Math.min(childOwners.length, ancestorOwners.length);
-    var i;
-
-    var deepestCommon = -1;
-    for (i = 0; i < minStackLen; i++) {
-      if (childOwners[i] === ancestorOwners[i]) {
-        deepestCommon = i;
-      } else {
-        break;
-      }
-    }
-
-    var UNKNOWN = '(unknown)';
-    var childOwnerNames = childOwners
-      .slice(deepestCommon + 1)
-      .map(inst => getComponentName(inst) || UNKNOWN);
-    var ancestorOwnerNames = ancestorOwners
-      .slice(deepestCommon + 1)
-      .map(inst => getComponentName(inst) || UNKNOWN);
-    var ownerInfo = []
-      .concat(
-        // If the parent and child instances have a common owner ancestor, start
-        // with that -- otherwise we just start with the parent's owners.
-        deepestCommon !== -1
-          ? getComponentName(childOwners[deepestCommon]) || UNKNOWN
-          : [],
-        ancestorOwnerNames,
-        ancestorTag,
-        // If we're warning about an invalid (non-parent) ancestry, add '...'
-        isParent ? [] : ['...'],
-        childOwnerNames,
-        childTag,
-      )
-      .join(' > ');
-
-    return ownerInfo;
-  };
-
   var didWarn = {};
 
-  validateDOMNesting = function(
-    childTag,
-    childText,
-    childInstance,
-    ancestorInfo,
-  ) {
+  validateDOMNesting = function(childTag, childText, ancestorInfo) {
     ancestorInfo = ancestorInfo || emptyAncestorInfo;
     var parentInfo = ancestorInfo.current;
     var parentTag = parentInfo && parentInfo.tag;
@@ -505,24 +430,8 @@ if (__DEV__) {
       return;
     }
 
-    var ancestorInstance = invalidParentOrAncestor.instance;
     var ancestorTag = invalidParentOrAncestor.tag;
-    var addendum;
-
-    if (childInstance != null) {
-      addendum =
-        ' See ' +
-        getOwnerInfo(
-          childInstance,
-          childTag,
-          ancestorInstance,
-          ancestorTag,
-          !!invalidParent,
-        ) +
-        '.';
-    } else {
-      addendum = getCurrentFiberStackAddendum();
-    }
+    var addendum = getCurrentFiberStackAddendum();
 
     var warnKey =
       !!invalidParent + '|' + childTag + '|' + ancestorTag + '|' + addendum;
diff --git a/src/renderers/dom/shared/warnValidStyle.js b/src/renderers/dom/shared/warnValidStyle.js
index 590c36f980db2..7a711e4a660cf 100644
--- a/src/renderers/dom/shared/warnValidStyle.js
+++ b/src/renderers/dom/shared/warnValidStyle.js
@@ -15,7 +15,6 @@ var warnValidStyle = emptyFunction;
 
 if (__DEV__) {
   var camelizeStyleName = require('fbjs/lib/camelizeStyleName');
-  var getComponentName = require('getComponentName');
   var warning = require('fbjs/lib/warning');
   var {getCurrentFiberOwnerName} = require('ReactDebugCurrentFiber');
 
@@ -30,7 +29,7 @@ if (__DEV__) {
   var warnedForNaNValue = false;
   var warnedForInfinityValue = false;
 
-  var warnHyphenatedStyleName = function(name, owner) {
+  var warnHyphenatedStyleName = function(name) {
     if (warnedStyleNames.hasOwnProperty(name) && warnedStyleNames[name]) {
       return;
     }
@@ -41,11 +40,11 @@ if (__DEV__) {
       'Unsupported style property %s. Did you mean %s?%s',
       name,
       camelizeStyleName(name),
-      checkRenderMessage(owner),
+      checkRenderMessage(),
     );
   };
 
-  var warnBadVendoredStyleName = function(name, owner) {
+  var warnBadVendoredStyleName = function(name) {
     if (warnedStyleNames.hasOwnProperty(name) && warnedStyleNames[name]) {
       return;
     }
@@ -56,11 +55,11 @@ if (__DEV__) {
       'Unsupported vendor-prefixed style property %s. Did you mean %s?%s',
       name,
       name.charAt(0).toUpperCase() + name.slice(1),
-      checkRenderMessage(owner),
+      checkRenderMessage(),
     );
   };
 
-  var warnStyleValueWithSemicolon = function(name, value, owner) {
+  var warnStyleValueWithSemicolon = function(name, value) {
     if (warnedStyleValues.hasOwnProperty(value) && warnedStyleValues[value]) {
       return;
     }
@@ -70,13 +69,13 @@ if (__DEV__) {
       false,
       "Style property values shouldn't contain a semicolon.%s " +
         'Try "%s: %s" instead.',
-      checkRenderMessage(owner),
+      checkRenderMessage(),
       name,
       value.replace(badStyleValueWithSemicolonPattern, ''),
     );
   };
 
-  var warnStyleValueIsNaN = function(name, value, owner) {
+  var warnStyleValueIsNaN = function(name, value) {
     if (warnedForNaNValue) {
       return;
     }
@@ -86,11 +85,11 @@ if (__DEV__) {
       false,
       '`NaN` is an invalid value for the `%s` css style property.%s',
       name,
-      checkRenderMessage(owner),
+      checkRenderMessage(),
     );
   };
 
-  var warnStyleValueIsInfinity = function(name, value, owner) {
+  var warnStyleValueIsInfinity = function(name, value) {
     if (warnedForInfinityValue) {
       return;
     }
@@ -100,21 +99,16 @@ if (__DEV__) {
       false,
       '`Infinity` is an invalid value for the `%s` css style property.%s',
       name,
-      checkRenderMessage(owner),
+      checkRenderMessage(),
     );
   };
 
-  var checkRenderMessage = function(owner) {
+  var checkRenderMessage = function() {
     var ownerName;
-    if (owner != null) {
-      // Stack passes the owner manually all the way to CSSPropertyOperations.
-      ownerName = getComponentName(owner);
-    } else {
-      // Fiber doesn't pass it but uses ReactDebugCurrentFiber to track it.
-      // It is only enabled in development and tracks host components too.
-      ownerName = getCurrentFiberOwnerName();
-      // TODO: also report the stack.
-    }
+    // Fiber doesn't pass it but uses ReactDebugCurrentFiber to track it.
+    // It is only enabled in development and tracks host components too.
+    ownerName = getCurrentFiberOwnerName();
+    // TODO: also report the stack.
     if (ownerName) {
       return '\n\nCheck the render method of `' + ownerName + '`.';
     }
@@ -122,24 +116,19 @@ if (__DEV__) {
   };
 
   warnValidStyle = function(name, value, component) {
-    var owner;
-    if (component) {
-      // TODO: this only works with Stack. Seems like we need to add unit tests?
-      owner = component._currentElement._owner;
-    }
     if (name.indexOf('-') > -1) {
-      warnHyphenatedStyleName(name, owner);
+      warnHyphenatedStyleName(name);
     } else if (badVendoredStyleNamePattern.test(name)) {
-      warnBadVendoredStyleName(name, owner);
+      warnBadVendoredStyleName(name);
     } else if (badStyleValueWithSemicolonPattern.test(value)) {
-      warnStyleValueWithSemicolon(name, value, owner);
+      warnStyleValueWithSemicolon(name, value);
     }
 
     if (typeof value === 'number') {
       if (isNaN(value)) {
-        warnStyleValueIsNaN(name, value, owner);
+        warnStyleValueIsNaN(name, value);
       } else if (!isFinite(value)) {
-        warnStyleValueIsInfinity(name, value, owner);
+        warnStyleValueIsInfinity(name, value);
       }
     }
   };
diff --git a/src/renderers/dom/test/ReactTestUtilsEntry.js b/src/renderers/dom/test/ReactTestUtilsEntry.js
index dd19f45b8881d..16247a23e9647 100644
--- a/src/renderers/dom/test/ReactTestUtilsEntry.js
+++ b/src/renderers/dom/test/ReactTestUtilsEntry.js
@@ -43,35 +43,6 @@ function Event(suffix) {}
  * @class ReactTestUtils
  */
 
-function findAllInRenderedStackTreeInternal(inst, test) {
-  if (!inst || !inst.getPublicInstance) {
-    return [];
-  }
-  var publicInst = inst.getPublicInstance();
-  var ret = test(publicInst) ? [publicInst] : [];
-  var currentElement = inst._currentElement;
-  if (ReactTestUtils.isDOMComponent(publicInst)) {
-    var renderedChildren = inst._renderedChildren;
-    var key;
-    for (key in renderedChildren) {
-      if (!renderedChildren.hasOwnProperty(key)) {
-        continue;
-      }
-      ret = ret.concat(
-        findAllInRenderedStackTreeInternal(renderedChildren[key], test),
-      );
-    }
-  } else if (
-    React.isValidElement(currentElement) &&
-    typeof currentElement.type === 'function'
-  ) {
-    ret = ret.concat(
-      findAllInRenderedStackTreeInternal(inst._renderedComponent, test),
-    );
-  }
-  return ret;
-}
-
 function findAllInRenderedFiberTreeInternal(fiber, test) {
   if (!fiber) {
     return [];
@@ -169,10 +140,7 @@ var ReactTestUtils = {
       return false;
     }
     var internalInstance = ReactInstanceMap.get(inst);
-    var constructor = typeof internalInstance.tag === 'number'
-      ? internalInstance.type // Fiber reconciler
-      : internalInstance._currentElement.type; // Stack reconciler
-
+    var constructor = internalInstance.type;
     return constructor === type;
   },
 
@@ -185,11 +153,7 @@ var ReactTestUtils = {
       'findAllInRenderedTree(...): instance must be a composite component',
     );
     var internalInstance = ReactInstanceMap.get(inst);
-    if (internalInstance && typeof internalInstance.tag === 'number') {
-      return findAllInRenderedFiberTreeInternal(internalInstance, test);
-    } else {
-      return findAllInRenderedStackTreeInternal(internalInstance, test);
-    }
+    return findAllInRenderedFiberTreeInternal(internalInstance, test);
   },
 
   /**
diff --git a/src/renderers/native/ReactNativeComponentTree.js b/src/renderers/native/ReactNativeComponentTree.js
index 0103da214859b..a67da6213bc68 100644
--- a/src/renderers/native/ReactNativeComponentTree.js
+++ b/src/renderers/native/ReactNativeComponentTree.js
@@ -14,41 +14,10 @@ var invariant = require('fbjs/lib/invariant');
 var instanceCache = {};
 var instanceProps = {};
 
-/**
- * Drill down (through composites and empty components) until we get a host or
- * host text component.
- *
- * This is pretty polymorphic but unavoidable with the current structure we have
- * for `_renderedChildren`.
- */
-function getRenderedHostOrTextFromComponent(component) {
-  var rendered;
-  while ((rendered = component._renderedComponent)) {
-    component = rendered;
-  }
-  return component;
-}
-
-/**
- * Populate `_hostNode` on the rendered host/text component with the given
- * DOM node. The passed `inst` can be a composite.
- */
-function precacheNode(inst, tag) {
-  var nativeInst = getRenderedHostOrTextFromComponent(inst);
-  instanceCache[tag] = nativeInst;
-}
-
 function precacheFiberNode(hostInst, tag) {
   instanceCache[tag] = hostInst;
 }
 
-function uncacheNode(inst) {
-  var tag = inst._rootNodeID;
-  if (tag) {
-    delete instanceCache[tag];
-  }
-}
-
 function uncacheFiberNode(tag) {
   delete instanceCache[tag];
   delete instanceProps[tag];
@@ -77,9 +46,7 @@ var ReactNativeComponentTree = {
   getInstanceFromNode: getInstanceFromTag,
   getNodeFromInstance: getTagFromInstance,
   precacheFiberNode,
-  precacheNode,
   uncacheFiberNode,
-  uncacheNode,
   getFiberCurrentPropsFromNode,
   updateFiberProps,
 };
diff --git a/src/renderers/native/__tests__/ReactNativeEvents-test.js b/src/renderers/native/__tests__/ReactNativeEvents-test.js
index 8c4cc735e0d07..d56d79d8407b9 100644
--- a/src/renderers/native/__tests__/ReactNativeEvents-test.js
+++ b/src/renderers/native/__tests__/ReactNativeEvents-test.js
@@ -250,9 +250,7 @@ it('handles when a responder is unmounted while a touch sequence is in progress'
     if (responder === null) {
       return null;
     }
-    const props = typeof responder.tag === 'number'
-      ? responder.memoizedProps
-      : responder._currentElement.props;
+    const props = responder.memoizedProps;
     return props ? props.id : null;
   }
 
@@ -342,9 +340,7 @@ it('handles events without target', () => {
     if (responder === null) {
       return null;
     }
-    const props = typeof responder.tag === 'number'
-      ? responder.memoizedProps
-      : responder._currentElement.props;
+    const props = responder.memoizedProps;
     return props ? props.id : null;
   }
 
diff --git a/src/renderers/shared/fiber/ReactChildFiber.js b/src/renderers/shared/fiber/ReactChildFiber.js
index bb5564f9f2c31..8eae6f410090d 100644
--- a/src/renderers/shared/fiber/ReactChildFiber.js
+++ b/src/renderers/shared/fiber/ReactChildFiber.js
@@ -13,7 +13,6 @@
 import type {ReactElement} from 'ReactElementType';
 import type {ReactCoroutine, ReactPortal, ReactYield} from 'ReactTypes';
 import type {Fiber} from 'ReactFiber';
-import type {ReactInstance} from 'ReactInstanceType';
 import type {PriorityLevel} from 'ReactPriorityLevel';
 
 var {REACT_COROUTINE_TYPE, REACT_YIELD_TYPE} = require('ReactCoroutine');
@@ -120,20 +119,15 @@ function coerceRef(current: Fiber | null, element: ReactElement) {
   let mixedRef = element.ref;
   if (mixedRef !== null && typeof mixedRef !== 'function') {
     if (element._owner) {
-      const owner: ?(Fiber | ReactInstance) = (element._owner: any);
+      const owner: ?Fiber = (element._owner: any);
       let inst;
       if (owner) {
-        if (typeof owner.tag === 'number') {
-          const ownerFiber = ((owner: any): Fiber);
-          invariant(
-            ownerFiber.tag === ClassComponent,
-            'Stateless function components cannot have refs.',
-          );
-          inst = ownerFiber.stateNode;
-        } else {
-          // Stack
-          inst = (owner: any).getPublicInstance();
-        }
+        const ownerFiber = ((owner: any): Fiber);
+        invariant(
+          ownerFiber.tag === ClassComponent,
+          'Stateless function components cannot have refs.',
+        );
+        inst = ownerFiber.stateNode;
       }
       invariant(
         inst,
diff --git a/src/renderers/shared/fiber/ReactDebugCurrentFiber.js b/src/renderers/shared/fiber/ReactDebugCurrentFiber.js
index 16a4096bd7282..92b3fa26f1441 100644
--- a/src/renderers/shared/fiber/ReactDebugCurrentFiber.js
+++ b/src/renderers/shared/fiber/ReactDebugCurrentFiber.js
@@ -29,8 +29,9 @@ function getCurrentFiberOwnerName(): string | null {
     if (fiber === null) {
       return null;
     }
-    if (fiber._debugOwner != null) {
-      return getComponentName(fiber._debugOwner);
+    const owner = fiber._debugOwner;
+    if (owner !== null && typeof owner !== 'undefined') {
+      return getComponentName(owner);
     }
   }
   return null;
diff --git a/src/renderers/shared/fiber/ReactFiber.js b/src/renderers/shared/fiber/ReactFiber.js
index 7fbb3a8af1170..dae77d55cc79b 100644
--- a/src/renderers/shared/fiber/ReactFiber.js
+++ b/src/renderers/shared/fiber/ReactFiber.js
@@ -11,7 +11,6 @@
 'use strict';
 
 import type {ReactElement, Source} from 'ReactElementType';
-import type {ReactInstance, DebugID} from 'ReactInstanceType';
 import type {
   ReactCoroutine,
   ReactFragment,
@@ -147,9 +146,9 @@ export type Fiber = {|
   // workInProgress : Fiber ->  alternate The alternate used for reuse happens
   // to be the same as work in progress.
   // __DEV__ only
-  _debugID?: DebugID,
+  _debugID?: number,
   _debugSource?: Source | null,
-  _debugOwner?: Fiber | ReactInstance | null, // Stack compatible
+  _debugOwner?: Fiber | null,
   _debugIsCurrentlyTiming?: boolean,
 |};
 
@@ -349,7 +348,7 @@ function createFiberFromElementType(
   type: mixed,
   key: null | string,
   internalContextTag: TypeOfInternalContext,
-  debugOwner: null | Fiber | ReactInstance,
+  debugOwner: null | Fiber,
 ): Fiber {
   let fiber;
   if (typeof type === 'function') {
diff --git a/src/renderers/shared/fiber/ReactFiberReconciler.js b/src/renderers/shared/fiber/ReactFiberReconciler.js
index 23738f3c40143..eafa87add0c78 100644
--- a/src/renderers/shared/fiber/ReactFiberReconciler.js
+++ b/src/renderers/shared/fiber/ReactFiberReconciler.js
@@ -25,7 +25,9 @@ var {
 } = require('ReactFiberContext');
 var {createFiberRoot} = require('ReactFiberRoot');
 var ReactFiberScheduler = require('ReactFiberScheduler');
+var ReactInstanceMap = require('ReactInstanceMap');
 var {HostComponent} = require('ReactTypeOfWork');
+var emptyObject = require('fbjs/lib/emptyObject');
 
 if (__DEV__) {
   var warning = require('fbjs/lib/warning');
@@ -39,8 +41,6 @@ var {
   findCurrentHostFiberWithNoPortals,
 } = require('ReactFiberTreeReflection');
 
-var getContextForSubtree = require('getContextForSubtree');
-
 export type Deadline = {
   timeRemaining: () => number,
 };
@@ -178,12 +178,19 @@ export type Reconciler<C, I, TI> = {
   findHostInstanceWithNoPortals(component: Fiber): I | TI | null,
 };
 
-getContextForSubtree._injectFiber(function(fiber: Fiber) {
+function getContextForSubtree(
+  parentComponent: ?React$Component<any, any>,
+): Object {
+  if (!parentComponent) {
+    return emptyObject;
+  }
+
+  const fiber = ReactInstanceMap.get(parentComponent);
   const parentContext = findCurrentUnmaskedContext(fiber);
   return isContextProvider(fiber)
     ? processChildContext(fiber, parentContext)
     : parentContext;
-});
+}
 
 module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   config: HostConfig<T, P, I, TI, PI, C, CX, PL>,
diff --git a/src/renderers/shared/hooks/ReactHostOperationHistoryHook.js b/src/renderers/shared/hooks/ReactHostOperationHistoryHook.js
deleted file mode 100644
index 4b6142ba2792d..0000000000000
--- a/src/renderers/shared/hooks/ReactHostOperationHistoryHook.js
+++ /dev/null
@@ -1,52 +0,0 @@
-/**
- * Copyright (c) 2016-present, Facebook, Inc.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- *
- * @providesModule ReactHostOperationHistoryHook
- * @flow
- */
-
-'use strict';
-
-import type {DebugID} from 'ReactInstanceType';
-
-export type Operation = {instanceID: DebugID} & (
-  | {type: 'mount', payload: string}
-  | {type: 'insert child', payload: {toIndex: number, content: string}}
-  | {type: 'move child', payload: {fromIndex: number, toIndex: number}}
-  | {type: 'replace children', payload: string}
-  | {type: 'replace text', payload: string}
-  | {type: 'replace with', payload: string}
-  | {type: 'update styles', payload: mixed /* Style Object */}
-  | {type: 'update attribute', payload: {[name: string]: string}}
-  | {type: 'remove attribute', payload: string});
-
-// Trust the developer to only use this with a __DEV__ check
-var ReactHostOperationHistoryHook = ((null: any): typeof ReactHostOperationHistoryHook);
-
-if (__DEV__) {
-  var history: Array<Operation> = [];
-
-  ReactHostOperationHistoryHook = {
-    onHostOperation(operation: Operation) {
-      history.push(operation);
-    },
-
-    clearHistory(): void {
-      if (ReactHostOperationHistoryHook._preventClearing) {
-        // Should only be used for tests.
-        return;
-      }
-
-      history = [];
-    },
-
-    getHistory(): Array<Operation> {
-      return history;
-    },
-  };
-}
-
-module.exports = ReactHostOperationHistoryHook;
diff --git a/src/renderers/shared/hooks/ReactInvalidSetStateWarningHook.js b/src/renderers/shared/hooks/ReactInvalidSetStateWarningHook.js
deleted file mode 100644
index eed4c066926cb..0000000000000
--- a/src/renderers/shared/hooks/ReactInvalidSetStateWarningHook.js
+++ /dev/null
@@ -1,39 +0,0 @@
-/**
- * Copyright (c) 2016-present, Facebook, Inc.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- *
- * @providesModule ReactInvalidSetStateWarningHook
- * @flow
- */
-
-'use strict';
-
-var ReactInvalidSetStateWarningHook = {};
-
-if (__DEV__) {
-  var warning = require('fbjs/lib/warning');
-  var processingChildContext = false;
-
-  var warnInvalidSetState = function() {
-    warning(
-      !processingChildContext,
-      'setState(...): Cannot call setState() inside getChildContext()',
-    );
-  };
-
-  ReactInvalidSetStateWarningHook = {
-    onBeginProcessingChildContext(): void {
-      processingChildContext = true;
-    },
-    onEndProcessingChildContext(): void {
-      processingChildContext = false;
-    },
-    onSetState(): void {
-      warnInvalidSetState();
-    },
-  };
-}
-
-module.exports = ReactInvalidSetStateWarningHook;
diff --git a/src/renderers/shared/shared/ReactTreeTraversal.js b/src/renderers/shared/shared/ReactTreeTraversal.js
index 1e73b8e199de6..8b58d7b10b758 100644
--- a/src/renderers/shared/shared/ReactTreeTraversal.js
+++ b/src/renderers/shared/shared/ReactTreeTraversal.js
@@ -12,21 +12,16 @@
 var {HostComponent} = require('ReactTypeOfWork');
 
 function getParent(inst) {
-  if (inst._hostParent !== undefined) {
-    return inst._hostParent;
-  }
-  if (typeof inst.tag === 'number') {
-    do {
-      inst = inst.return;
-      // TODO: If this is a HostRoot we might want to bail out.
-      // That is depending on if we want nested subtrees (layers) to bubble
-      // events to their parent. We could also go through parentNode on the
-      // host node but that wouldn't work for React Native and doesn't let us
-      // do the portal feature.
-    } while (inst && inst.tag !== HostComponent);
-    if (inst) {
-      return inst;
-    }
+  do {
+    inst = inst.return;
+    // TODO: If this is a HostRoot we might want to bail out.
+    // That is depending on if we want nested subtrees (layers) to bubble
+    // events to their parent. We could also go through parentNode on the
+    // host node but that wouldn't work for React Native and doesn't let us
+    // do the portal feature.
+  } while (inst && inst.tag !== HostComponent);
+  if (inst) {
+    return inst;
   }
   return null;
 }
diff --git a/src/renderers/shared/shared/event/EventPluginHub.js b/src/renderers/shared/shared/event/EventPluginHub.js
index 975166553b613..7238b3ec773a0 100644
--- a/src/renderers/shared/shared/event/EventPluginHub.js
+++ b/src/renderers/shared/shared/event/EventPluginHub.js
@@ -122,43 +122,20 @@ var EventPluginHub = {
 
     // TODO: shouldPreventMouseEvent is DOM-specific and definitely should not
     // live here; needs to be moved to a better place soon
-    if (typeof inst.tag === 'number') {
-      const stateNode = inst.stateNode;
-      if (!stateNode) {
-        // Work in progress (ex: onload events in incremental mode).
-        return null;
-      }
-      const props = EventPluginUtils.getFiberCurrentPropsFromNode(stateNode);
-      if (!props) {
-        // Work in progress.
-        return null;
-      }
-      listener = props[registrationName];
-      if (shouldPreventMouseEvent(registrationName, inst.type, props)) {
-        return null;
-      }
-    } else {
-      const currentElement = inst._currentElement;
-      if (
-        typeof currentElement === 'string' ||
-        typeof currentElement === 'number'
-      ) {
-        // Text node, let it bubble through.
-        return null;
-      }
-      if (!inst._rootNodeID) {
-        // If the instance is already unmounted, we have no listeners.
-        return null;
-      }
-      const props = currentElement.props;
-      listener = props[registrationName];
-      if (
-        shouldPreventMouseEvent(registrationName, currentElement.type, props)
-      ) {
-        return null;
-      }
+    const stateNode = inst.stateNode;
+    if (!stateNode) {
+      // Work in progress (ex: onload events in incremental mode).
+      return null;
+    }
+    const props = EventPluginUtils.getFiberCurrentPropsFromNode(stateNode);
+    if (!props) {
+      // Work in progress.
+      return null;
+    }
+    listener = props[registrationName];
+    if (shouldPreventMouseEvent(registrationName, inst.type, props)) {
+      return null;
     }
-
     invariant(
       !listener || typeof listener === 'function',
       'Expected `%s` listener to be a function, instead got a value of `%s` type.',
diff --git a/src/renderers/shared/shared/event/PluginModuleType.js b/src/renderers/shared/shared/event/PluginModuleType.js
index c6e0bcf4ad13f..ad9b90e78d735 100644
--- a/src/renderers/shared/shared/event/PluginModuleType.js
+++ b/src/renderers/shared/shared/event/PluginModuleType.js
@@ -10,7 +10,7 @@
 
 'use strict';
 
-import type {ReactInstance} from 'ReactInstanceType';
+import type {Fiber} from 'ReactFiber';
 import type {
   DispatchConfig,
   ReactSyntheticEvent,
@@ -26,7 +26,7 @@ export type PluginModule<NativeEvent> = {
   eventTypes: EventTypes,
   extractEvents: (
     topLevelType: string,
-    targetInst: ReactInstance,
+    targetInst: Fiber,
     nativeTarget: NativeEvent,
     nativeEventTarget: EventTarget,
   ) => null | ReactSyntheticEvent,
diff --git a/src/renderers/shared/shared/event/ReactControlledComponent.js b/src/renderers/shared/shared/event/ReactControlledComponent.js
index 520a4e8e65190..45eec0c89eb6d 100644
--- a/src/renderers/shared/shared/event/ReactControlledComponent.js
+++ b/src/renderers/shared/shared/event/ReactControlledComponent.js
@@ -36,30 +36,20 @@ function restoreStateOfTarget(target) {
     // Unmounted
     return;
   }
-  if (typeof internalInstance.tag === 'number') {
-    invariant(
-      fiberHostComponent &&
-        typeof fiberHostComponent.restoreControlledState === 'function',
-      'Fiber needs to be injected to handle a fiber target for controlled ' +
-        'events. This error is likely caused by a bug in React. Please file an issue.',
-    );
-    const props = EventPluginUtils.getFiberCurrentPropsFromNode(
-      internalInstance.stateNode,
-    );
-    fiberHostComponent.restoreControlledState(
-      internalInstance.stateNode,
-      internalInstance.type,
-      props,
-    );
-    return;
-  }
   invariant(
-    typeof internalInstance.restoreControlledState === 'function',
-    'The internal instance must be a React host component. ' +
-      'This error is likely caused by a bug in React. Please file an issue.',
+    fiberHostComponent &&
+      typeof fiberHostComponent.restoreControlledState === 'function',
+    'Fiber needs to be injected to handle a fiber target for controlled ' +
+      'events. This error is likely caused by a bug in React. Please file an issue.',
+  );
+  const props = EventPluginUtils.getFiberCurrentPropsFromNode(
+    internalInstance.stateNode,
+  );
+  fiberHostComponent.restoreControlledState(
+    internalInstance.stateNode,
+    internalInstance.type,
+    props,
   );
-  // If it is not a Fiber, we can just use dynamic dispatch.
-  internalInstance.restoreControlledState();
 }
 
 var ReactControlledComponent = {
diff --git a/src/renderers/shared/shared/event/ReactSyntheticEventType.js b/src/renderers/shared/shared/event/ReactSyntheticEventType.js
index f60f3d670aecf..86c8a711aeb58 100644
--- a/src/renderers/shared/shared/event/ReactSyntheticEventType.js
+++ b/src/renderers/shared/shared/event/ReactSyntheticEventType.js
@@ -12,7 +12,7 @@
 
 'use strict';
 
-import type {ReactInstance} from 'ReactInstanceType';
+import type {Fiber} from 'ReactFiber';
 
 export type DispatchConfig = {
   dependencies: Array<string>,
@@ -27,7 +27,7 @@ export type ReactSyntheticEvent = {
   dispatchConfig: DispatchConfig,
   getPooled: (
     dispatchConfig: DispatchConfig,
-    targetInst: ReactInstance,
+    targetInst: Fiber,
     nativeTarget: Event,
     nativeEventTarget: EventTarget,
   ) => ReactSyntheticEvent,
diff --git a/src/renderers/shared/shared/event/eventPlugins/__tests__/ResponderEventPlugin-test.js b/src/renderers/shared/shared/event/eventPlugins/__tests__/ResponderEventPlugin-test.js
index cb6fafdb7764c..90a7ad6d43865 100644
--- a/src/renderers/shared/shared/event/eventPlugins/__tests__/ResponderEventPlugin-test.js
+++ b/src/renderers/shared/shared/event/eventPlugins/__tests__/ResponderEventPlugin-test.js
@@ -9,6 +9,8 @@
 
 'use strict';
 
+var {HostComponent} = require('ReactTypeOfWork');
+
 var EventPluginHub;
 var ResponderEventPlugin;
 
@@ -318,35 +320,37 @@ var PARENT_HOST_NODE = {};
 var CHILD_HOST_NODE = {};
 var CHILD_HOST_NODE2 = {};
 
+// These intentionally look like Fibers. ReactTreeTraversal depends on their field names.
+// TODO: we could test this with regular DOM nodes (and real fibers) instead.
 var GRANDPARENT_INST = {
-  _hostParent: null,
-  _rootNodeID: '1',
-  _hostNode: GRANDPARENT_HOST_NODE,
-  _currentElement: {props: {}},
+  return: null,
+  tag: HostComponent,
+  stateNode: GRANDPARENT_HOST_NODE,
+  memoizedProps: {},
 };
 var PARENT_INST = {
-  _hostParent: GRANDPARENT_INST,
-  _rootNodeID: '2',
-  _hostNode: PARENT_HOST_NODE,
-  _currentElement: {props: {}},
+  return: GRANDPARENT_INST,
+  tag: HostComponent,
+  stateNode: PARENT_HOST_NODE,
+  memoizedProps: {},
 };
 var CHILD_INST = {
-  _hostParent: PARENT_INST,
-  _rootNodeID: '3',
-  _hostNode: CHILD_HOST_NODE,
-  _currentElement: {props: {}},
+  return: PARENT_INST,
+  tag: HostComponent,
+  stateNode: CHILD_HOST_NODE,
+  memoizedProps: {},
 };
 var CHILD_INST2 = {
-  _hostParent: PARENT_INST,
-  _rootNodeID: '4',
-  _hostNode: CHILD_HOST_NODE2,
-  _currentElement: {props: {}},
+  return: PARENT_INST,
+  tag: HostComponent,
+  stateNode: CHILD_HOST_NODE2,
+  memoizedProps: {},
 };
 
-GRANDPARENT_HOST_NODE._reactInstance = GRANDPARENT_INST;
-PARENT_HOST_NODE._reactInstance = PARENT_INST;
-CHILD_HOST_NODE._reactInstance = CHILD_INST;
-CHILD_HOST_NODE2._reactInstance = CHILD_INST2;
+GRANDPARENT_HOST_NODE.testInstance = GRANDPARENT_INST;
+PARENT_HOST_NODE.testInstance = PARENT_INST;
+CHILD_HOST_NODE.testInstance = CHILD_INST;
+CHILD_HOST_NODE2.testInstance = CHILD_INST2;
 
 var three = {
   grandParent: GRANDPARENT_HOST_NODE,
@@ -361,19 +365,23 @@ var siblings = {
 };
 
 function getInstanceFromNode(node) {
-  return node._reactInstance;
+  return node.testInstance;
 }
 
 function getNodeFromInstance(inst) {
-  return inst._hostNode;
+  return inst.stateNode;
+}
+
+function getFiberCurrentPropsFromNode(node) {
+  return node.testInstance.memoizedProps;
 }
 
-function putListener(node, registrationName, handler) {
-  node._currentElement.props[registrationName] = handler;
+function putListener(instance, registrationName, handler) {
+  instance.memoizedProps[registrationName] = handler;
 }
 
-function deleteAllListeners(node) {
-  node._currentElement.props = {};
+function deleteAllListeners(instance) {
+  instance.memoizedProps = {};
 }
 
 describe('ResponderEventPlugin', () => {
@@ -398,6 +406,7 @@ describe('ResponderEventPlugin', () => {
     injectComponentTree({
       getInstanceFromNode,
       getNodeFromInstance,
+      getFiberCurrentPropsFromNode,
     });
   });
 
diff --git a/src/renderers/shared/shared/getContextForSubtree.js b/src/renderers/shared/shared/getContextForSubtree.js
deleted file mode 100644
index 027e63a2e5f69..0000000000000
--- a/src/renderers/shared/shared/getContextForSubtree.js
+++ /dev/null
@@ -1,39 +0,0 @@
-/**
- * Copyright (c) 2013-present, Facebook, Inc.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- *
- * @providesModule getContextForSubtree
- * @flow
- */
-
-const ReactInstanceMap = require('ReactInstanceMap');
-
-const emptyObject = require('fbjs/lib/emptyObject');
-const invariant = require('fbjs/lib/invariant');
-
-let getContextFiber = function(arg) {
-  invariant(false, 'Missing injection for fiber getContextForSubtree');
-};
-
-function getContextForSubtree(
-  parentComponent: ?React$Component<any, any>,
-): Object {
-  if (!parentComponent) {
-    return emptyObject;
-  }
-
-  const instance = ReactInstanceMap.get(parentComponent);
-  if (typeof instance.tag === 'number') {
-    return getContextFiber(instance);
-  } else {
-    return instance._processChildContext(instance._context);
-  }
-}
-
-getContextForSubtree._injectFiber = function(fn) {
-  getContextFiber = fn;
-};
-
-module.exports = getContextForSubtree;
diff --git a/src/shared/ReactInstanceType.js b/src/shared/ReactInstanceType.js
deleted file mode 100644
index ed29794f1ba32..0000000000000
--- a/src/shared/ReactInstanceType.js
+++ /dev/null
@@ -1,27 +0,0 @@
-/**
- * Copyright (c) 2016-present, Facebook, Inc.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- *
- * @providesModule ReactInstanceType
- * @flow
- */
-
-'use strict';
-
-// TODO: delete this file.
-// It was only necessary for Stack.
-
-import type {ReactElement} from 'ReactElementType';
-
-export type DebugID = number;
-
-export type ReactInstance = {
-  mountComponent: any,
-  unmountComponent: any,
-  receiveComponent: any,
-  getName: () => string,
-  getPublicInstance: any,
-  _currentElement: ReactElement,
-};
diff --git a/src/shared/utils/getComponentName.js b/src/shared/utils/getComponentName.js
index 772949d19d599..5a3b00ec29f90 100644
--- a/src/shared/utils/getComponentName.js
+++ b/src/shared/utils/getComponentName.js
@@ -10,27 +10,15 @@
 
 'use strict';
 
-import type {ReactInstance} from 'ReactInstanceType';
 import type {Fiber} from 'ReactFiber';
 
-function getComponentName(
-  instanceOrFiber: ReactInstance | Fiber,
-): string | null {
-  if (typeof instanceOrFiber.getName === 'function') {
-    // Stack reconciler
-    const instance = ((instanceOrFiber: any): ReactInstance);
-    return instance.getName();
+function getComponentName(fiber: Fiber): string | null {
+  const {type} = fiber;
+  if (typeof type === 'string') {
+    return type;
   }
-  if (typeof instanceOrFiber.tag === 'number') {
-    // Fiber reconciler
-    const fiber = ((instanceOrFiber: any): Fiber);
-    const {type} = fiber;
-    if (typeof type === 'string') {
-      return type;
-    }
-    if (typeof type === 'function') {
-      return type.displayName || type.name;
-    }
+  if (typeof type === 'function') {
+    return type.displayName || type.name;
   }
   return null;
 }