anchor.diff:  ///
   /// class _ColorfulButtonState extends State<ColorfulButton> {
   ///   FocusNode _node;
  +///   bool _focused = false;
   ///   FocusAttachment _nodeAttachment;
   ///   Color _color = Colors.white;
   ///
  
positive.code: ///
  ///   @override
  ///   void dispose() {
  ///     // The attachment will automatically be detached in dispose().
  ///     _node.dispose();
  ///     super.dispose();
  
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) {
  