anchor.diff:    }
   
     // Returns the child which should be retained as the child of this node.
  -  UINode _syncChild(UINode node, UINode oldNode, RenderCSSContainer host,
  -      RenderCSS insertBefore) {
  -
  +  UINode _syncChild(UINode node, UINode oldNode, dynamic slot) {
  +    assert(node != null);
       assert(oldNode == null || node._key == oldNode._key);
   
       if (node == oldNode) {
  
positive.code:       return;
  
      _trace('$_key rebuilding...');
      _sync(null, _host, _root);
    }
  }
  
  
negative.code: 
      if (node._willSync(oldNode)) {
        _traceSync(_SyncOperation.STATEFUL, node._key);
        oldNode._sync(node, slot);
        node._defunct = true;
        assert(oldNode._root is RenderCSS);
        return oldNode;
  