anchor.diff:      key: key,
       initialScrollOffset: initialScrollOffset,
       scrollDirection: scrollDirection,
  +    onScroll: onScroll,
       itemsWrap: itemsWrap,
       itemExtent: itemExtent,
       padding: padding);
  
positive.code:     Key key,
      double initialScrollOffset,
      ScrollDirection scrollDirection: ScrollDirection.vertical,
      this.items,
      this.itemBuilder,
      itemsWrap: false,
  
negative.code: 
    final double initialScrollOffset;
    final ScrollDirection scrollDirection;
    final ScrollListener onScroll;
  }
  
  abstract class ScrollableState<T extends Scrollable> extends State<T> {
  