anchor.diff:      _currentlyBuilding = null;
       _currentOrder = lastOrder;
   
  -    _built = _syncChild(_built, oldBuilt, slot);
  +    _built = syncChild(_built, oldBuilt, slot);
       _dirty = false;
       root = _built.root;
       assert(root != null);
  
positive.code:   }
  
    // Returns the child which should be retained as the child of this node.
    UINode _syncChild(UINode node, UINode oldNode, dynamic slot) {
      assert(node != null);
      assert(oldNode == null || node._key == oldNode._key);
  
      if (node == oldNode) {
        _traceSync(_SyncOperation.IDENTICAL, node._key);
        return node; // Nothing to do. Subtrees must be identical.
      }
  
      // TODO(rafaelw): This eagerly removes the old DOM. It may be that a
      // new component was built that could re-use some of it. Consider
  
negative.code:     UINode oldNode = null;
  
      void sync(int atIndex) {
        children[atIndex] = syncChild(currentNode, oldNode, nextSibling);
        assert(children[atIndex] != null);
      }
  
  