[ReactNative] Remove global MutationObserver to fix Bluebird feature …

…detection

Summary:
Bluebird is checking if MutationObserver exists and if it does, uses it and instant crashes. Since we don't have the DOM, MutationObserver doesn't make sense.

Also, pretty cool thing, the bluebird maintainer updated the feature detection to fallback to setImmediate before checking setTimeout. He committed and made a new version in less than 5 minutes!

facebook#335 (comment)

Test Plan: Run bluebird and make sure it runs with setImmediate

Libraries/JavaScriptAppEngine/Initialization/InitializeJavaScriptAppEngine.js

@@ -60,6 +60,10 @@ function setupDocumentShim() {
   if (GLOBAL.document) {
     GLOBAL.document.createElement = null;
   }
+
+  // There is no DOM so MutationObserver doesn't make sense. It is used
+  // as feature detection in Bluebird Promise implementation
+  GLOBAL.MutationObserver = undefined;
 }
 
 function handleErrorWithRedBox(e) {