switched fast-fs to use fs for API calls

Summary:
When debugging graceful-fs I noticed that I don't get close() calls https://github.com/isaacs/node-graceful-fs/blob/master/graceful-fs.js#L37.
This is because fastfs uses fs returned from graceful-fs https://github.com/facebook/react-native/blob/master/packager/react-packager/src/node-haste/fastfs.js#L14 and graceful-fs returns close() of a clone object https://github.com/isaacs/node-graceful-fs/blob/master/graceful-fs.js#L27 that is not patched by graceful-fs yet.

Reviewed By: cpojer

Differential Revision: D4021446

fbshipit-source-id: 87e50ff779dfea13b8db2b97575bff0bfd7778eb

packager/react-packager/src/node-haste/fastfs.js

@@ -11,7 +11,7 @@
 const denodeify = require('denodeify');
 const {EventEmitter} = require('events');
 
-const fs = require('graceful-fs');
+const fs = require('fs');
 const path = require('./fastpath');
 
 const readFile = denodeify(fs.readFile);