anchor.diff:  typedef Widget RouteBuilder(RouteArguments args);
   typedef RouteBuilder RouteGenerator(String name);
   typedef void StateRouteCallback(StateRoute route);
  -typedef void NotificationCallback();
  +typedef void _RouteCallback(Route route);
   
   class Navigator extends StatefulComponent {
     Navigator({
  
positive.code:         currentRoute.didPop(null);
          _currentPosition -= 1;
        }
        _history.insert(_currentPosition + 1, route);
        _currentPosition += 1;
      });
    }
  
  
negative.code:   bool get isActuallyOpaque => (performance == null || _performance.isCompleted) && opaque;
  
    Widget build(NavigatorState navigator, PerformanceView nextRoutePerformance);
  
    void didPush() {
      _performance?.forward();
    }
  
    void didPop([dynamic result]) {
      _performance?.reverse();
      if (performance == null && _onRemoveRoute != null)
        _onRemoveRoute(this);
    }
  
    String toString() => '$runtimeType()';
  