Fix use strict warnings

src/core/ReactComponentEnvironment.js

@@ -15,6 +15,7 @@
  *
  * @providesModule ReactComponentEnvironment
  */
+"use strict";
 
 var ReactComponentBrowserEnvironment =
   require('ReactComponentBrowserEnvironment');

src/core/ReactMultiChildUpdateTypes.js

@@ -15,6 +15,7 @@
  *
  * @providesModule ReactMultiChildUpdateTypes
  */
+"use strict";
 
 var keyMirror = require('keyMirror');