anchor.diff:    }) : super(key: key);
   
     RenderNode createNode();
  -  RenderNodeWrapper get emptyNode;
   
     void insert(RenderNodeWrapper child, dynamic slot);
   
  
positive.code:       var ancestor = findAncestor(RenderNodeWrapper);
        if (ancestor is RenderNodeWrapper)
          ancestor.insert(this, slot);
        old = emptyNode;
      } else {
        root = old.root;
        assert(root != null);
  
negative.code:     var startIndex = 0;
      var endIndex = children.length;
  
      var oldChildren = old == null ? [] : old.children;
      var oldStartIndex = 0;
      var oldEndIndex = oldChildren.length;
  
  