* This caches "child event subscribers" by key and removes them when they're not needed anymore. Once a navigator unmounts we also remove upstream subscribers.
* Fix tests
* Add fbemitter, keep flow passing
* Begin support for event emitter
- Adds emitter to navigation prop
- Emits top level onAction event
- stub getChildEventSubscriber for child events
* Support navigationState.isNavigating
* Focus and blur events starting to work
- Navigation completion action wired up
- Event chaining logic built in getChildEventSubscriber
- Renamed onAction evt to ‘action’
* Wrap up events progress and testing
* Rename to isTransitioning and COMPLETE_TRANSITION
* rm accidental dependency
* Suppoert event payload type