anchor.diff:  }
   
   class FlexContainer extends OneChildListRenderNodeWrapper {
  -
     RenderFlex root;
     RenderFlex createNode() => new RenderFlex(direction: this.direction);
  -
  -  static final FlexContainer _emptyContainer = new FlexContainer();
  -    // direction doesn't matter if it's empty
  -
  -  RenderNodeWrapper get emptyNode => _emptyContainer;
   
     final FlexDirection direction;
   
  
positive.code:     var startIndex = 0;
      var endIndex = children.length;
  
      var oldChildren = old.children;
      var oldStartIndex = 0;
      var oldEndIndex = oldChildren.length;
  
  
negative.code:     Object key,
      List<UINode> children,
      this.direction: FlexDirection.Horizontal
    }) : super(key: key, children: children);
  
    void syncRenderNode(UINode old) {
      super.syncRenderNode(old);
  