anchor.diff:  class ScrollableList<T> extends FixedHeightScrollable {
     ScrollableList({
       Key key,
  -    ScrollDirection scrollDirection,
  +    ScrollDirection scrollDirection: ScrollDirection.vertical,
       this.items,
       this.itemBuilder,
       double itemExtent,
  
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:     this.scrollDirection: ScrollDirection.vertical
    }) : super(key: key) {
      assert(scrollDirection == ScrollDirection.vertical ||
             scrollDirection == ScrollDirection.horizontal);
    }
  
    ScrollDirection scrollDirection;
  