Type ReactComponentEnvironment (#7566)

(cherry picked from commit f784a2d50a)

src/renderers/shared/stack/reconciler/ReactComponentEnvironment.js

@@ -7,12 +7,21 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactComponentEnvironment
+ * @flow
  */
 
 'use strict';
 
 var invariant = require('invariant');
 
+type ReplaceNodeWithMarkup = (node: HTMLElement, markup: string) => void;
+type ProcessChildrenUpdates = (instance: mixed, updates: mixed) => void;
+
+type Environment = {
+  replaceNodeWithMarkup: ReplaceNodeWithMarkup,
+  processChildrenUpdates: ProcessChildrenUpdates,
+};
+
 var injected = false;
 
 var ReactComponentEnvironment = {
@@ -21,16 +30,16 @@ var ReactComponentEnvironment = {
    * Optionally injectable hook for swapping out mount images in the middle of
    * the tree.
    */
-  replaceNodeWithMarkup: null,
+  replaceNodeWithMarkup: (null: ?ReplaceNodeWithMarkup),
 
   /**
    * Optionally injectable hook for processing a queue of child updates. Will
    * later move into MultiChildComponents.
    */
-  processChildrenUpdates: null,
+  processChildrenUpdates: (null: ?ReplaceNodeWithMarkup),
 
   injection: {
-    injectEnvironment: function(environment) {
+    injectEnvironment: function(environment: Environment) {
       invariant(
         !injected,
         'ReactCompositeComponent: injectEnvironment() can only be called once.'