Added ReactNative feature-flag switch between Stack and Fiber builds

Reviewed By: sebmarkbage

Differential Revision: D4606164

fbshipit-source-id: 5f25dbc52298d359e11e52341ff3570c797a6ea9

Author: Brian Vaughn

Libraries/Renderer/src/renderers/native/ReactNative.js

@@ -11,5 +11,8 @@
  */
 'use strict';
 
-// TODO (bvaughn) Enable Fiber experiement via ReactNativeFeatureFlags
-module.exports = require('ReactNativeStack');
+const ReactNativeFeatureFlags = require('ReactNativeFeatureFlags');
+
+module.exports = ReactNativeFeatureFlags.useFiber
+  ? require('ReactNativeFiber')
+  : require('ReactNativeStack')