anchor.diff:  
     void _sync(UINode old, dynamic slot) {
       UINode oldContent = old == null ? null : (old as ContentNode).content;
  -    content = _syncChild(content, oldContent, slot);
  +    content = syncChild(content, oldContent, slot);
       assert(content.root != null);
       root = content.root;
     }
  
positive.code:     UINode oldNode = null;
  
      void sync(int atIndex) {
        children[atIndex] = _syncChild(currentNode, oldNode, nextSibling);
        assert(children[atIndex] != null);
      }
  
  
negative.code:     _currentlyBuilding = null;
      _currentOrder = lastOrder;
  
      _built = syncChild(_built, oldBuilt, slot);
      _dirty = false;
      root = _built.root;
      assert(root != null);
  