diff --git a/src/renderers/art/ReactART.js b/src/renderers/art/ReactART.js
index 0f102926e31ba..bf1e298ef0184 100644
--- a/src/renderers/art/ReactART.js
+++ b/src/renderers/art/ReactART.js
@@ -93,7 +93,7 @@ function injectAfter(parentNode, referenceNode, node) {
 
 // ContainerMixin for components that can hold ART nodes
 
-const ContainerMixin = assign({}, ReactMultiChild, {
+const ContainerMixin = assign({}, ReactMultiChild.prototype, {
 
   /**
    * Moves a child component to the supplied index.
diff --git a/src/renderers/dom/shared/ReactDOMComponent.js b/src/renderers/dom/shared/ReactDOMComponent.js
index c9bae5a8026aa..3429c287638a6 100644
--- a/src/renderers/dom/shared/ReactDOMComponent.js
+++ b/src/renderers/dom/shared/ReactDOMComponent.js
@@ -1200,7 +1200,7 @@ ReactDOMComponent.Mixin = {
 Object.assign(
   ReactDOMComponent.prototype,
   ReactDOMComponent.Mixin,
-  ReactMultiChild
+  ReactMultiChild.prototype
 );
 
 module.exports = ReactDOMComponent;
diff --git a/src/renderers/native/ReactNativeBaseComponent.js b/src/renderers/native/ReactNativeBaseComponent.js
index 2192b019fc4ab..1872386b6896a 100644
--- a/src/renderers/native/ReactNativeBaseComponent.js
+++ b/src/renderers/native/ReactNativeBaseComponent.js
@@ -224,7 +224,7 @@ ReactNativeBaseComponent.Mixin = {
  */
 Object.assign(
   ReactNativeBaseComponent.prototype,
-  ReactMultiChild,
+  ReactMultiChild.prototype,
   ReactNativeBaseComponent.Mixin,
   NativeMethodsMixin
 );
diff --git a/src/renderers/shared/stack/reconciler/ReactMultiChild.js b/src/renderers/shared/stack/reconciler/ReactMultiChild.js
index 220de49f3e6b5..d567c96573aea 100644
--- a/src/renderers/shared/stack/reconciler/ReactMultiChild.js
+++ b/src/renderers/shared/stack/reconciler/ReactMultiChild.js
@@ -169,8 +169,8 @@ if (__DEV__) {
  * children. This is used by `ReactDOMComponent` to mount, update, and
  * unmount child components.
  */
-var ReactMultiChild = {
-  _reconcilerInstantiateChildren: function(nestedChildren, transaction, context) {
+class ReactMultiChild {
+  _reconcilerInstantiateChildren(nestedChildren, transaction, context) {
     if (__DEV__) {
       var selfDebugID = getDebugID(this);
       if (this._currentElement) {
@@ -187,9 +187,9 @@ var ReactMultiChild = {
     return ReactChildReconciler.instantiateChildren(
       nestedChildren, transaction, context
     );
-  },
+  }
 
-  _reconcilerUpdateChildren: function(
+  _reconcilerUpdateChildren(
     prevChildren,
     nextNestedChildrenElements,
     mountImages,
@@ -235,7 +235,7 @@ var ReactMultiChild = {
       selfDebugID
     );
     return nextChildren;
-  },
+  }
 
   /**
    * Generates a "mount image" for each of the supplied children. In the case
@@ -245,7 +245,7 @@ var ReactMultiChild = {
    * @return {array} An array of mounted representations.
    * @internal
    */
-  mountChildren: function(nestedChildren, transaction, context) {
+  mountChildren(nestedChildren, transaction, context) {
     var children = this._reconcilerInstantiateChildren(
       nestedChildren, transaction, context
     );
@@ -278,7 +278,7 @@ var ReactMultiChild = {
     }
 
     return mountImages;
-  },
+  }
 
   /**
    * Replaces any rendered children with a text content string.
@@ -286,7 +286,7 @@ var ReactMultiChild = {
    * @param {string} nextContent String of content.
    * @internal
    */
-  updateTextContent: function(nextContent) {
+  updateTextContent(nextContent) {
     var prevChildren = this._renderedChildren;
     // Remove any rendered children.
     ReactChildReconciler.unmountChildren(prevChildren, false);
@@ -298,7 +298,7 @@ var ReactMultiChild = {
     // Set new text content.
     var updates = [makeTextContent(nextContent)];
     processQueue(this, updates);
-  },
+  }
 
   /**
    * Replaces any rendered children with a markup string.
@@ -306,7 +306,7 @@ var ReactMultiChild = {
    * @param {string} nextMarkup String of markup.
    * @internal
    */
-  updateMarkup: function(nextMarkup) {
+  updateMarkup(nextMarkup) {
     var prevChildren = this._renderedChildren;
     // Remove any rendered children.
     ReactChildReconciler.unmountChildren(prevChildren, false);
@@ -317,7 +317,7 @@ var ReactMultiChild = {
     }
     var updates = [makeSetMarkup(nextMarkup)];
     processQueue(this, updates);
-  },
+  }
 
   /**
    * Updates the rendered children with new children.
@@ -326,10 +326,10 @@ var ReactMultiChild = {
    * @param {ReactReconcileTransaction} transaction
    * @internal
    */
-  updateChildren: function(nextNestedChildrenElements, transaction, context) {
+  updateChildren(nextNestedChildrenElements, transaction, context) {
     // Hook used by React ART
     this._updateChildren(nextNestedChildrenElements, transaction, context);
-  },
+  }
 
   /**
    * @param {?object} nextNestedChildrenElements Nested child element maps.
@@ -337,7 +337,7 @@ var ReactMultiChild = {
    * @final
    * @protected
    */
-  _updateChildren: function(nextNestedChildrenElements, transaction, context) {
+  _updateChildren(nextNestedChildrenElements, transaction, context) {
     var prevChildren = this._renderedChildren;
     var removedNodes = {};
     var mountImages = [];
@@ -414,7 +414,7 @@ var ReactMultiChild = {
     if (__DEV__) {
       setChildrenForInstrumentation.call(this, nextChildren);
     }
-  },
+  }
 
   /**
    * Unmounts all rendered children. This should be used to clean up children
@@ -423,11 +423,11 @@ var ReactMultiChild = {
    *
    * @internal
    */
-  unmountChildren: function(safely) {
+  unmountChildren(safely) {
     var renderedChildren = this._renderedChildren;
     ReactChildReconciler.unmountChildren(renderedChildren, safely);
     this._renderedChildren = null;
-  },
+  }
 
   /**
    * Moves a child component to the supplied index.
@@ -437,14 +437,14 @@ var ReactMultiChild = {
    * @param {number} lastIndex Last index visited of the siblings of `child`.
    * @protected
    */
-  moveChild: function(child, afterNode, toIndex, lastIndex) {
+  moveChild(child, afterNode, toIndex, lastIndex) {
     // If the index of `child` is less than `lastIndex`, then it needs to
     // be moved. Otherwise, we do not need to move it because a child will be
     // inserted or moved before `child`.
     if (child._mountIndex < lastIndex) {
       return makeMove(child, afterNode, toIndex);
     }
-  },
+  }
 
   /**
    * Creates a child component.
@@ -453,9 +453,9 @@ var ReactMultiChild = {
    * @param {string} mountImage Markup to insert.
    * @protected
    */
-  createChild: function(child, afterNode, mountImage) {
+  createChild(child, afterNode, mountImage) {
     return makeInsertMarkup(mountImage, afterNode, child._mountIndex);
-  },
+  }
 
   /**
    * Removes a child component.
@@ -463,9 +463,9 @@ var ReactMultiChild = {
    * @param {ReactComponent} child Child to remove.
    * @protected
    */
-  removeChild: function(child, node) {
+  removeChild(child, node) {
     return makeRemove(child, node);
-  },
+  }
 
   /**
    * Mounts a child with the supplied name.
@@ -478,7 +478,7 @@ var ReactMultiChild = {
    * @param {ReactReconcileTransaction} transaction
    * @private
    */
-  _mountChildAtIndex: function(
+  _mountChildAtIndex(
     child,
     mountImage,
     afterNode,
@@ -487,7 +487,7 @@ var ReactMultiChild = {
     context) {
     child._mountIndex = index;
     return this.createChild(child, afterNode, mountImage);
-  },
+  }
 
   /**
    * Unmounts a rendered child.
@@ -497,11 +497,11 @@ var ReactMultiChild = {
    * @param {ReactComponent} child Component to unmount.
    * @private
    */
-  _unmountChild: function(child, node) {
+  _unmountChild(child, node) {
     var update = this.removeChild(child, node);
     child._mountIndex = null;
     return update;
-  },
+  }
 };
 
 module.exports = ReactMultiChild;
diff --git a/src/renderers/shared/stack/reconciler/ReactOwner.js b/src/renderers/shared/stack/reconciler/ReactOwner.js
index 1e9b411f9d0ff..a804513d73583 100644
--- a/src/renderers/shared/stack/reconciler/ReactOwner.js
+++ b/src/renderers/shared/stack/reconciler/ReactOwner.js
@@ -15,6 +15,7 @@
 var invariant = require('invariant');
 
 import type { ReactInstance } from 'ReactInstanceType';
+import type { Transaction } from 'Transaction';
 
 /**
  * @param {?object} object
@@ -73,7 +74,7 @@ var ReactOwner = {
     component: ReactInstance,
     ref: string,
     owner: ReactInstance,
-    transaction,
+    transaction: Transaction,
   ): void {
     invariant(
       isValidOwner(owner),
diff --git a/src/renderers/testing/ReactTestEmptyComponent.js b/src/renderers/testing/ReactTestEmptyComponent.js
index 9f6659ea8dcd9..067d5e8e043bc 100644
--- a/src/renderers/testing/ReactTestEmptyComponent.js
+++ b/src/renderers/testing/ReactTestEmptyComponent.js
@@ -13,6 +13,8 @@
 'use strict';
 
 class ReactTestEmptyComponent {
+  _currentElement: null;
+  
   constructor() {
     this._currentElement = null;
   }
diff --git a/src/renderers/testing/ReactTestMount.js b/src/renderers/testing/ReactTestMount.js
index 71811e4bfa890..f13e5d20f6d7f 100644
--- a/src/renderers/testing/ReactTestMount.js
+++ b/src/renderers/testing/ReactTestMount.js
@@ -20,8 +20,10 @@ var getHostComponentFromComposite = require('getHostComponentFromComposite');
 var instantiateReactComponent = require('instantiateReactComponent');
 var invariant = require('invariant');
 
-type TestRendererOptions = {
-  createNodeMock: (element: ReactElement) => Object,
+import type { ReactElement } from 'ReactElementType';
+
+export type TestRendererOptions = {
+  createNodeMock: (element: ReactElement) => any,
 };
 
 var defaultTestOptions = {
diff --git a/src/renderers/testing/ReactTestReconcileTransaction.js b/src/renderers/testing/ReactTestReconcileTransaction.js
index ba87192001b52..50585e08be106 100644
--- a/src/renderers/testing/ReactTestReconcileTransaction.js
+++ b/src/renderers/testing/ReactTestReconcileTransaction.js
@@ -16,6 +16,8 @@ var PooledClass = require('PooledClass');
 var Transaction = require('Transaction');
 var ReactUpdateQueue = require('ReactUpdateQueue');
 
+import type { TestRendererOptions } from 'ReactTestMount';
+
 /**
  * Provides a `CallbackQueue` queue for collecting `onDOMReady` callbacks during
  * the performing of the transaction.
@@ -57,7 +59,7 @@ var TRANSACTION_WRAPPERS = [ON_DOM_READY_QUEUEING];
  *
  * @class ReactTestReconcileTransaction
  */
-function ReactTestReconcileTransaction(testOptions) {
+function ReactTestReconcileTransaction(testOptions: TestRendererOptions) {
   this.reinitializeTransaction();
   this.testOptions = testOptions;
   this.reactMountReady = CallbackQueue.getPooled(this);
diff --git a/src/renderers/testing/ReactTestRenderer.js b/src/renderers/testing/ReactTestRenderer.js
index a7daf6ebf91ec..a60707da6643b 100644
--- a/src/renderers/testing/ReactTestRenderer.js
+++ b/src/renderers/testing/ReactTestRenderer.js
@@ -24,11 +24,13 @@ var ReactTestTextComponent = require('ReactTestTextComponent');
 var ReactTestEmptyComponent = require('ReactTestEmptyComponent');
 
 import type { ReactElement } from 'ReactElementType';
+import type { ReactInstance } from 'ReactInstanceType';
 
 type ReactTestRendererJSON = {
   type: string,
   props: { [propName: string]: string },
-  children: Array<string | ReactTestRendererJSON>,
+  children: null | Array<string | ReactTestRendererJSON>,
+  $$typeof?: any
 }
 
 /**
@@ -46,8 +48,13 @@ function getRenderedHostOrTextFromComponent(component) {
   return component;
 }
 
-class ReactTestComponent {
+class ReactTestComponent extends ReactMultiChild {
+  _currentElement: ReactElement;
+  _renderedChildren: null | Object;
+  _topLevelWrapper: null | ReactInstance;
+
   constructor(element: ReactElement) {
+    super();
     this._currentElement = element;
     this._renderedChildren = null;
     this._topLevelWrapper = null;
@@ -89,7 +96,7 @@ class ReactTestComponent {
         childrenJSON.push(json);
       }
     }
-    var object = {
+    var object: ReactTestRendererJSON = {
       type: this._currentElement.type,
       props: props,
       children: childrenJSON.length ? childrenJSON : null,
@@ -104,8 +111,6 @@ class ReactTestComponent {
   unmountComponent(): void {}
 }
 
-Object.assign(ReactTestComponent.prototype, ReactMultiChild);
-
 // =============================================================================
 
 ReactUpdates.injection.injectReconcileTransaction(
diff --git a/src/renderers/testing/ReactTestTextComponent.js b/src/renderers/testing/ReactTestTextComponent.js
index 18e22d4e9f543..a6e61a655dcae 100644
--- a/src/renderers/testing/ReactTestTextComponent.js
+++ b/src/renderers/testing/ReactTestTextComponent.js
@@ -15,6 +15,8 @@
 import type { ReactText } from 'ReactTypes';
 
 class ReactTestTextComponent {
+  _currentElement: ReactText;
+  
   constructor(element: ReactText) {
     this._currentElement = element;
   }