anchor.diff:  
     void _handleDragEnd(Offset velocity) {
       fling(velocity);
  -  }
  -
  -  final List<ScrollListener> _listeners = new List<ScrollListener>();
  -  void addListener(ScrollListener listener) {
  -    _listeners.add(listener);
  -  }
  -
  -  void removeListener(ScrollListener listener) {
  -    _listeners.remove(listener);
  -  }
  -
  -  void _notifyListeners() {
  -    List<ScrollListener> localListeners = new List<ScrollListener>.from(_listeners);
  -    for (ScrollListener listener in localListeners)
  -      listener();
     }
   }
   
  
positive.code:   ScrollableMixedWidgetList({
      Key key,
      double initialScrollOffset,
      this.builder,
      this.token,
      this.onInvalidatorAvailable
    }) : super(key: key, initialScrollOffset: initialScrollOffset);
  
    final IndexedBuilder builder;
    final Object token;
  
negative.code:     Key key,
      double initialScrollOffset,
      ScrollDirection scrollDirection: ScrollDirection.vertical,
      ScrollListener onScroll,
      this.items,
      this.itemBuilder,
      itemsWrap: false,
  