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:         assert(!route.modal);
          continue;
        }
        route.ensurePerformance(
          direction: (i <= _currentPosition) ? AnimationDirection.forward : AnimationDirection.reverse
        );
        route._onDismissed = () {
          assert(_history.contains(route));
          if (_history.lastIndexOf(route) <= _currentPosition)
            popRoute(route);
        };
        route._onRemoveRoute = () {
          assert(_history.contains(route));
          setState(() {
            _history.remove(route);
          });
        };
        visibleRoutes.add(
          new KeyedSubtree(
            key: new ObjectKey(route),
  
negative.code:     push(new PageRoute(builder));
    }
  
    void _insertRoute(Route route) {
      _history.insert(_currentPosition, route);
      route._onDismissed = _handleRouteDismissed;
      route._onRemoveRoute = _handleRemoveRoute;
      route.didPush();
    }
  
    void push(Route route) {
      assert(!_debugCurrentlyHaveRoute(route));
      setState(() {
  