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:   }
  
    // 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:     _currentlyBuilding = null;
      _currentOrder = lastOrder;
  
      _built = syncChild(_built, oldBuilt, slot);
      _dirty = false;
      root = _built.root;
      assert(root != null);
  