Summary:
So far, XHR only supports a few `onfoo` event handlers, not the entier `EventTarget` interface (`addEventListener`, `removeEventListener`). It also doesn't support the `upload` object on Android -- for no good reason. Even if we don't send any events there yet, there's no reason we have to break consuming code that wants to register an event handler there. This PR rectifies all that.
Fortunately, adding proper `EventTarget` support is very easy thanks to `event-target-shim`. We already use it in our WebSocket implementation. It transparently handles the `addEventListener('foo', ...)` as well as `onfoo` APIs, so when you dispatch an event on the event target, the right handlers will be invoked. The event object is wrapped so that `event.target` is set properly. Basically, it's a super easy way to make us conform to the spec.
Also added a bit of polish here and there, using ES2015 class property goodness to consolidate a lot of Flow property definitions with the corresponding property initializers.
**T
Closes https://github.com/facebook/react-native/pull/7017
Reviewed By: fkgozali
Differential Revision: D3202021
Pulled By: martinbigio
fb-gh-sync-id: 2b007682074356c75c774fab337672918b6c4355
fbshipit-source-id: 2b007682074356c75c774fab337672918b6c4355
b5f14ea8f1