anchor.diff:      this.scrollDirection: ScrollDirection.vertical
     }) : super(key: key) {
       assert(scrollDirection == ScrollDirection.vertical ||
  -        scrollDirection == ScrollDirection.horizontal);
  +           scrollDirection == ScrollDirection.horizontal);
     }
   
     ScrollDirection scrollDirection;
  
positive.code: // about offscreen widgets consuming resources.
  abstract class FixedHeightScrollable extends Scrollable {
  
    FixedHeightScrollable({ Key key, ScrollDirection scrollDirection, this.itemExtent, this.padding })
        : super(key: key, scrollDirection: scrollDirection) {
      assert(itemExtent != null);
    }
  
  
negative.code: class ScrollableList<T> extends FixedHeightScrollable {
    ScrollableList({
      Key key,
      ScrollDirection scrollDirection: ScrollDirection.vertical,
      this.items,
      this.itemBuilder,
      double itemExtent,
  