Summary:
I read my code and one thing stroke me: What if the same event emitter dispatches several events during the same event beat? In the previous implementation, only the first one will be delivered because the first `release` call consumes stored strong reference.
So, I changed the API a bit to make this more explicit: we have `retain` & `release` methods and we have a getter that works (multiple times) only if the object was successfully retained.
Reviewed By: sahrens
Differential Revision: D13668147
fbshipit-source-id: c00af5f15bc7e30aa704d46bd23584b918b6f75a
8d83d5f8eb