void addObserver(PageVisibilityObserver observer, Route<dynamic> route) {
  assert(observer != null);
  assert(route != null);
  final observers =
      _listeners.putIfAbsent(route, () => <PageVisibilityObserver>{});
  observers.add(observer);
  Logger.log(
      'page_visibility, #addObserver, $observers, ${route.settings.name}');
}