diff --git a/src/shared/utils/ReactFeatureFlags.js b/src/shared/utils/ReactFeatureFlags.js
index 30b5f4c7de2e..5b3bb1b457bd 100644
--- a/src/shared/utils/ReactFeatureFlags.js
+++ b/src/shared/utils/ReactFeatureFlags.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactFeatureFlags
+ * @flow
  */
 
 'use strict';
diff --git a/src/shared/utils/ReactNodeTypes.js b/src/shared/utils/ReactNodeTypes.js
index 7457b92661e5..2b5e40376d7a 100644
--- a/src/shared/utils/ReactNodeTypes.js
+++ b/src/shared/utils/ReactNodeTypes.js
@@ -7,10 +7,13 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactNodeTypes
+ * @flow
  */
 
 'use strict';
 
+type ReactNodeType = 0 | 1 | 2;
+
 var ReactElement = require('ReactElement');
 
 var invariant = require('invariant');
@@ -20,7 +23,7 @@ var ReactNodeTypes = {
   COMPOSITE: 1,
   EMPTY: 2,
 
-  getType: function(node) {
+  getType: function(node: ReactElement<any>): ReactNodeType {
     if (node === null || node === false) {
       return ReactNodeTypes.EMPTY;
     } else if (ReactElement.isValidElement(node)) {
diff --git a/src/shared/utils/flattenChildren.js b/src/shared/utils/flattenChildren.js
index b529f25c90ff..afb12d5a4655 100644
--- a/src/shared/utils/flattenChildren.js
+++ b/src/shared/utils/flattenChildren.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule flattenChildren
+ * @flow
  */
 
 'use strict';
@@ -22,22 +23,29 @@ var warning = require('warning');
  * @param {!string} name String name of key path to child.
  * @param {number=} selfDebugID Optional debugID of the current internal instance.
  */
-function flattenSingleChildIntoContext(traverseContext, child, name, selfDebugID) {
+function flattenSingleChildIntoContext(
+  traverseContext: mixed,
+  child: ReactElement<any>,
+  name: string,
+  selfDebugID: number
+): void {
   // We found a component instance.
-  var result = traverseContext;
-  var keyUnique = (result[name] === undefined);
-  if (__DEV__) {
-    warning(
-      keyUnique,
-      'flattenChildren(...): Encountered two children with the same key, ' +
-      '`%s`. Child keys must be unique; when two children share a key, only ' +
-      'the first child will be used.%s',
-      KeyEscapeUtils.unescape(name),
-      ReactComponentTreeDevtool.getStackAddendumByID(selfDebugID)
-    );
-  }
-  if (keyUnique && child != null) {
-    result[name] = child;
+  if (traverseContext && typeof traverseContext === 'object') {
+    const result = traverseContext;
+    const keyUnique = (result[name] === undefined);
+    if (__DEV__) {
+      warning(
+        keyUnique,
+        'flattenChildren(...): Encountered two children with the same key, ' +
+        '`%s`. Child keys must be unique; when two children share a key, only ' +
+        'the first child will be used.%s',
+        KeyEscapeUtils.unescape(name),
+        ReactComponentTreeDevtool.getStackAddendumByID(selfDebugID)
+      );
+    }
+    if (keyUnique && child != null) {
+      result[name] = child;
+    }
   }
 }
 
@@ -46,7 +54,7 @@ function flattenSingleChildIntoContext(traverseContext, child, name, selfDebugID
  * children will not be included in the resulting object.
  * @return {!object} flattened children keyed by name.
  */
-function flattenChildren(children, selfDebugID) {
+function flattenChildren(children: ReactElement<any>, selfDebugID: number): ?{ [name: string]: ReactElement<any> } {
   if (children == null) {
     return children;
   }
diff --git a/src/shared/utils/getIteratorFn.js b/src/shared/utils/getIteratorFn.js
index 5da5c4846a0c..4ed780aa204e 100644
--- a/src/shared/utils/getIteratorFn.js
+++ b/src/shared/utils/getIteratorFn.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule getIteratorFn
+ * @flow
  */
 
 'use strict';
@@ -29,7 +30,7 @@ var FAUX_ITERATOR_SYMBOL = '@@iterator'; // Before Symbol spec.
  * @param {?object} maybeIterable
  * @return {?function}
  */
-function getIteratorFn(maybeIterable) {
+function getIteratorFn(maybeIterable: ?any): ?(p: ReactElement<any>) => void {
   var iteratorFn = maybeIterable && (
     (ITERATOR_SYMBOL && maybeIterable[ITERATOR_SYMBOL]) ||
     maybeIterable[FAUX_ITERATOR_SYMBOL]
diff --git a/src/shared/utils/reactProdInvariant.js b/src/shared/utils/reactProdInvariant.js
index ca235bbeb9d2..ed61b59585ce 100644
--- a/src/shared/utils/reactProdInvariant.js
+++ b/src/shared/utils/reactProdInvariant.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule reactProdInvariant
+ * @flow
  */
 'use strict';
 
@@ -16,7 +17,7 @@
  * and will _only_ be required by the corresponding babel pass.
  * It always throws.
  */
-function reactProdInvariant(code) {
+function reactProdInvariant(code: string): void {
   var argCount = arguments.length - 1;
 
   var message = (
@@ -33,7 +34,7 @@ function reactProdInvariant(code) {
     ' for full errors and additional helpful warnings.'
   );
 
-  var error = new Error(message);
+  var error: Error & { framesToPop?: number } = new Error(message);
   error.name = 'Invariant Violation';
   error.framesToPop = 1; // we don't care about reactProdInvariant's own frame