anchor.diff:      key: key,
       initialScrollOffset: initialScrollOffset,
       scrollDirection: scrollDirection,
  +    onScroll: onScroll,
       items: items,
       itemBuilder: itemBuilder,
       itemsWrap: itemsWrap,
  
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,
      double initialScrollOffset,
      ScrollDirection scrollDirection: ScrollDirection.horizontal,
      ScrollListener onScroll,
      List<T> items,
      ItemBuilder<T> itemBuilder,
      bool itemsWrap: false,
  