anchor.diff:    }
   
     Widget _build(int index) {
  -    return _childWidgets.putIfAbsent(index, () {
  -      Widget child = widget.delegate.build(this, index);
  -      if (child == null)
  -        return null;
  -      return new RepaintBoundary.wrap(child, index);
  -    });
  +    return _childWidgets.putIfAbsent(index, () => widget.delegate.build(this, index));
     }
   
     @override
  
positive.code:   ) => null;
  
    bool shouldRebuild(@checked SliverChildDelegate oldDelegate);
  }
  
  // ///
  
negative.code:     assert(children != null);
      if (index < 0 || index >= children.length)
        return null;
      final Widget child = children[index];
      assert(child != null);
      return new RepaintBoundary.wrap(child, index);
    }
  
    @override
  