Plugins in standalone flutter apps typically live as long as the application is alive. However, in add2app use case, we would want to be able to free the resources and destroy the plugin if the isolate backing the plugin is no longer active (e.g. if FlutterView is destroyed). In some cases, this can be done by subscribing to the activity lifecycle (e.g. if the Flutter is added as a separate activity) but most often the plugin is attached to a single view.

We need a way to detect that the isolate (i.e. FlutterNativeView) is being destroyed.