Fix jest test that runs the polyfill 10 times

Summary: @​public

jest is running the polyfill multiple times on the same environment (cc @cpojer, need to fix that!). By default jest doesn't have XMLHttpRequest polyfilled so it'll define a property with writable to be false. It'll fatal the second time it tries to override XMLHttpRequest.

The hacky workaround is to make properties that do not exist with writable: true. But the long term fix would be to make jest stop running the polyfill multiple times.

Reviewed By: @javache

Differential Revision: D2532019

fb-gh-sync-id: a82abf69541781a64a0744798c736f90833e28cb

Author: vjeux

Libraries/JavaScriptAppEngine/Initialization/InitializeJavaScriptAppEngine.js

@@ -46,26 +46,33 @@ function handleError(e, isFatal) {
 
 /**
  * Assigns a new global property, replacing the existing one if there is one.
- * 
+ *
  * Existing properties are preserved as `originalPropertyName`. Both properties
  * will maintain the same enumerability & configurability.
- * 
+ *
  * This allows you to undo the more aggressive polyfills, should you need to.
  * For example, if you want to route network requests through DevTools (to trace
  * them):
  *
- *     GLOBAL.XMLHTTPRequest = GLOBAL.originalXMLHTTPRequest;
- * 
+ *     global.XMLHttpRequest = global.originalXMLHttpRequest;
+ *
  * For more info on that particular case, see:
  * https://github.com/facebook/react-native/issues/934
  */
 function polyfillGlobal(name, newValue, scope=GLOBAL) {
-  var descriptor = Object.getOwnPropertyDescriptor(scope, name);
+  var descriptor = Object.getOwnPropertyDescriptor(scope, name) || {
+    // jest for some bad reasons runs the polyfill code multiple times. In jest
+    // environment, XmlHttpRequest doesn't exist so getOwnPropertyDescriptor
+    // returns undefined and defineProperty default for writable is false.
+    // Therefore, the second time it runs, defineProperty will fatal :(
+    writable: true,
+  };
 
   if (scope[name] !== undefined) {
     var backupName = `original${name[0].toUpperCase()}${name.substr(1)}`;
     Object.defineProperty(scope, backupName, {...descriptor, value: scope[name]});
   }
+
   Object.defineProperty(scope, name, {...descriptor, value: newValue});
 }