anchor.diff:    }
   }
   
  -class TreeViewItem<T extends TreeNode<T>> extends StatefulWidget {
  -  const TreeViewItem(
  +class _TreeViewItem<T extends TreeNode<T>> extends StatefulWidget {
  +  const _TreeViewItem(
       this.data, {
  -    this.buildDisplay,
  -    this.onItemExpanded,
  -    this.onItemSelected,
  +    required this.buildDisplay,
  +    required this.onItemExpanded,
  +    required this.onItemSelected,
     });
   
     final T data;
  
positive.code: // Copyright 2020 The Chromium Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  // @dart=2.9
  
  import 'dart:async';
  
  
negative.code:   _TreeViewItemState<T> createState() => _TreeViewItemState<T>();
  }
  
  class _TreeViewItemState<T extends TreeNode<T>> extends State<_TreeViewItem<T>>
      with TickerProviderStateMixin, CollapsibleAnimationMixin {
    @override
    Widget build(BuildContext context) {
  