Event handlers from different component trees should batch together

[sophiebits]

Right now, ReactUpdates.batchedUpdates is called in ReactEventEmitterMixin.js but should probably be higher up in ReactTopLevelEventCallback.js so that with nested roots, updates are queued into the same batch.

[syranide]

Side-note, I'm not 100% up on the details of state and DOM batching. But it seems like RAF-batching is something they intend to make default "soon in the future", which would take care of batching both state and DOM updates I believe? Again, in the future.

[sophiebits]

Yeah, we're going to do that once we figure out how pending state works. I believe we still want to flush any updates within the same event loop though, so this change will still be helpful.