A callback that add a new Route to the provided navigator.
The callback must use one of the API methods on the NavigatorState that have "restorable" in their name (e.g. NavigatorState.restorablePush, NavigatorState.restorablePushNamed, etc.) and return the opaque ID returned by those methods.
This callback is invoked when present is called with the
Object that was passed to that method and the NavigatorState obtained
final RoutePresentationCallback onPresent;