Use fetch as a polyfill

Summary:
The old whatwg-fetch module doesn't actually export anything, so we would always hit the `else` condition.

The new whatwg-fetch (3.0.0, introduced in facebook#24418) now exports an ES module. As a result, `whatwg` and `whatwg.fetch` are both truthy but the `module.exports` will end up empty. This breaks the RN fetch module.

This will switch the behavior back to the expected polyfill behavior (calling `require('whatwg-fetch')` and allowing it to polyfill fetch in global scope). The RN fetch module will re-export these globals.

Reviewed By: cpojer

Differential Revision: D15639851

fbshipit-source-id: ebd8bce85f7797d8539f53982e515ac47f6425e7

Author: Arthur Lee

Libraries/Network/fetch.js

@@ -11,10 +11,8 @@
 
 'use strict';
 
-const whatwg = require('whatwg-fetch');
+// side-effectful require() to put fetch,
+// Headers, Request, Response in global scope
+require('whatwg-fetch');
 
-if (whatwg && whatwg.fetch) {
-  module.exports = whatwg;
-} else {
-  module.exports = {fetch, Headers, Request, Response};
-}
+module.exports = {fetch, Headers, Request, Response};