anchor.diff:      PageRoute route = new PageRoute(config.routes['/']);
       assert(route != null);
       assert(!route.ephemeral);
  -    _history.add(route);
  +    _insertRoute(route);
     }
   
     void pushState(State owner, Function callback) {
  
positive.code:   bool _debugCurrentlyHaveRoute(Route route) {
      int index = _history.indexOf(route);
      return index >= 0 && index <= _currentPosition;
    }
  
    Widget build(BuildContext context) {
  
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()';
  