anchor.diff:  }
   
   class HistoryEntry {
  -  HistoryEntry(this.route);
  +  HistoryEntry({ this.route, this.key });
     final RouteBase route;
  +  final int key;
     // TODO(jackson): Keep track of the requested transition
   }
   
  
positive.code:   }
  
    void push(RouteBase route) {
      HistoryEntry historyEntry = new HistoryEntry(route);
      history.insert(historyIndex + 1, historyEntry);
      historyIndex++;
    }
  
negative.code:       }
        if (content == null)
          continue;
        Transition transition = new Transition(
          key: historyEntry.key.toString(),
          content: content,
          direction: (i <= state.historyIndex) ? TransitionDirection.forward : TransitionDirection.reverse,
          interactive: (i == state.historyIndex),
  