anchor.diff:  ///
   ///   @override
   ///   void dispose() {
  +///     _node.removeListener(_handleFocusChange);
   ///     // The attachment will automatically be detached in dispose().
   ///     _node.dispose();
   ///     super.dispose();
  
positive.code: ///
  /// class _ColorfulButtonState extends State<ColorfulButton> {
  ///   FocusNode _node;
  ///   FocusAttachment _nodeAttachment;
  ///   Color _color = Colors.white;
  ///
  
negative.code: ///   void initState() {
  ///     super.initState();
  ///     _node = FocusNode(debugLabel: 'Button');
  ///     _node.addListener(_handleFocusChange);
  ///     _nodeAttachment = _node.attach(context, onKey: _handleKeyPress);
  ///   }
  ///
  ///   void _handleFocusChange() {
  ///     if (_node.hasFocus != _focused) {
  ///       setState(() {
  ///         _focused = _node.hasFocus;
  ///       });
  ///     }
  ///   }
  ///
  ///   bool _handleKeyPress(FocusNode node, RawKeyEvent event) {
  