anchor.diff:      if (oldNode == null) {
         _trace('_sync(insert) ${node._key}');
       } else {
  -      _trace('_sync(stateless) ${node._key} <- ${oldNode._key}');
  +      _trace('_sync(stateless) ${node._key}');
       }
       node._sync(oldNode, host, insertBefore);
       if (oldNode != null)
  
positive.code:   Node _syncChild(Node node, Node oldNode, sky.ParentNode host,
        sky.Node insertBefore) {
  
      if (node == oldNode) {
        _trace('_sync(identical) ${node._key}');
        return node; // Nothing to do. Subtrees must be identical.
  
negative.code:     }
  
      if (node._willSync(oldNode)) {
        _trace('_sync(statefull) ${node._key}');
        oldNode._sync(node, host, insertBefore);
        node._defunct = true;
        assert(oldNode._root is sky.Node);
  