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: const double _kMinFlingVelocity = -kMaxFlingVelocity * _kMillisecondsPerSecond;
  const double _kMaxFlingVelocity = kMaxFlingVelocity * _kMillisecondsPerSecond;
  
  typedef void ScrollListener();
  
  /// A base class for scrollable widgets that reacts to user input and generates
  /// a scrollOffset.
  
negative.code:     key: key,
      initialScrollOffset: initialScrollOffset,
      scrollDirection: scrollDirection,
      onScroll: onScroll,
      items: items,
      itemBuilder: itemBuilder,
      itemsWrap: itemsWrap,
  