Fabric: Simplified relationship between RawEvent and EventEmitter

Summary: Instead of relying on explicit `RawEventDispatchable` function, we simply check the existence of the `weak_ptr` to `EventTarget`. This is efficient and sufficient because only an EventEmitter retains an associated EventTarget.

Reviewed By: mdvacca

Differential Revision: D9764858

fbshipit-source-id: 4ac25d925f189d0f8b9002e52388fd51629934a8

ReactCommon/fabric/events/RawEvent.cpp

@@ -13,17 +13,11 @@ namespace react {
 RawEvent::RawEvent(
   std::string type,
   folly::dynamic payload,
-  WeakEventTarget eventTarget,
-  RawEventDispatchable isDispatchable
+  WeakEventTarget eventTarget
 ):
   type(std::move(type)),
   payload(std::move(payload)),
-  eventTarget(std::move(eventTarget)),
-  isDispatchable_(std::move(isDispatchable)) {}
-
-bool RawEvent::isDispatchable() const {
-  return isDispatchable_();
-}
+  eventTarget(std::move(eventTarget)) {}
 
 } // namespace react
 } // namespace facebook