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