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: 
    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:   }
  
    // Returns the child which should be retained as the child of this node.
    UINode syncChild(UINode node, UINode oldNode, dynamic slot) {
  
      if (node == oldNode) {
        _traceSync(_SyncOperation.IDENTICAL, node == null ? '*null*' : node._key);
        return node; // Nothing to do. Subtrees must be identical.
      }
  
      if (node == null) {
        // the child in this slot has gone away
        removeChild(oldNode);
        return null;
      }
      assert(oldNode == null || node._key == oldNode._key);
  
      // 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
  