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