The UIDartState is now always owned by the isolate and always freed in
the isolate cleanup callback.
In the isolate shutdown callback, if the isolate being shut down is the
main isolate, the RuntimeController is informed which in turn notifies
the RuntimeHolder and thus the ApplicationControllerImpl. The
ApplicationControllerImpl tears down the whole Flutter application.
This fixes Fuchsia bug: MI4-328
d917c35e72