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