anchor.diff:  const Point _kTransitionStartPoint = const Point(0.0, 100.0);
   enum TransitionDirection { forward, reverse }
   class Transition extends AnimatedComponent {
  -  Transition({ this.content, this.direction, this.onDismissed, this.interactive });
  +  Transition({
  +    String key,
  +    this.content,
  +    this.direction,
  +    this.onDismissed,
  +    this.interactive
  +  }) : super(key: key);
     Widget content;
     TransitionDirection direction;
     bool interactive;
  
positive.code:   Widget build() {
      List<Widget> visibleRoutes = new List<Widget>();
      for (int i = 0; i < state.history.length; i++) {
        HistoryEntry historyEntry = state.history[i];
        Widget content = historyEntry.route.build(this, historyEntry.route);
        if (i == 0) {
  
negative.code:       }
        if (content == null)
          continue;
        String key = historyEntry.route.key;
        Transition transition = new Transition(content: content, key: key)
          ..direction = (i <= state.historyIndex) ? TransitionDirection.forward : TransitionDirection.reverse
          ..interactive = (i == state.historyIndex)
          ..onDismissed = () {
  