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:     assert(children != null);
      if (index < 0 || index >= children.length)
        return null;
      return children[index];
    }
  
    @override
  
negative.code:   ) => null;
  
    bool shouldRebuild(@checked SliverChildDelegate oldDelegate);
  }
  
  class SliverChildBuilderDelegate extends SliverChildDelegate {
    const SliverChildBuilderDelegate(this.builder, { this.childCount });
  
    final IndexedWidgetBuilder builder;
  
    final int childCount;
  
    @override
    Widget build(BuildContext context, int index) {
      assert(builder != null);
      if (index < 0 || (childCount != null && index >= childCount))
        return null;
      final Widget child = builder(context, index);
      if (child == null)
        return null;
      return new RepaintBoundary.wrap(child, index);
    }
  
    @override
    int get estimatedChildCount => childCount;
  
    @override
    bool shouldRebuild(@checked SliverChildListDelegate oldDelegate) => true;
  }
  
  // ///
  