anchor.diff:          HashMap.fromIterables(allClasses, allClasses.map(getInstanceFieldMap));
   
       for (var class_ in allClasses) {
  -      Set<Class> superclasses;
  +      Set<Class>? superclasses;
   
         // Visit accessors in the current class, and see if they override an
         // otherwise private field.
  
positive.code:         // Look in all super classes to see if we're overriding a field in our
          // library, if so mark that field as overridden.
          var name = member.name.text;
          _overriddenPrivateFields.addAll(superclasses
              .map((c) => allFields[c][name])
              .where((f) => f != null));
        }
      }
    }
  
negative.code: // for details. All rights reserved. Use of this source code is governed by a
  // BSD-style license that can be found in the LICENSE file.
  
  import 'dart:collection' show HashMap, HashSet, Queue;
  
  import 'package:collection/collection.dart' show IterableNullableExtension;
  import 'package:kernel/core_types.dart';
  import 'package:kernel/kernel.dart';
  import 'package:kernel/type_environment.dart';
  