anchor.diff:      this._fallbacks = const <Type, Generator>{},
     ]);
   
  -  final String name;
  -  final AppContext _parent;
  +  final String? name;
  +  final AppContext? _parent;
     final Map<Type, Generator> _overrides;
     final Map<Type, Generator> _fallbacks;
     final Map<Type, dynamic> _values = <Type, dynamic>{};
   
  -  List<Type> _reentrantChecks;
  +  List<Type>? _reentrantChecks;
   
     /// Bootstrap context.
     static final AppContext _root = AppContext._(null, 'ROOT');
  
positive.code: // Copyright 2014 The Flutter 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 = 2.8
  
  import 'dart:async';
  import 'dart:collection';
  
negative.code:   /// name. This is useful for debugging purposes and is analogous to naming a
    /// thread in Java.
    Future<V> run<V>({
      required FutureOr<V> Function() body,
      String? name,
      Map<Type, Generator>? overrides,
      Map<Type, Generator>? fallbacks,
      ZoneSpecification? zoneSpecification,
    }) async {
      final AppContext child = AppContext._(
        this,
  