Summary:
In preparation for upcoming changes, it is useful / necessary to have a CustomEvent and Event polyfill for React Native.
In browser environments, both of those are expected to be accessible in the global scope, so we do the same here.
Changelog: [Added][JS] Event and CustomEvent W3C-compatible polyfills
Reviewed By: necolas
Differential Revision: D34462447
fbshipit-source-id: 5efdad6f24c268a6d248d4e3351fc96715ee3fdf