anchor.diff:      // If the field was marked non-virtual, we know for sure.
       if (field.isStatic) return false;
   
  -    var class_ = field.enclosingClass;
  +    var class_ = field.enclosingClass!;
       if (class_.isEnum) {
         // Enums are not extensible.
         return false;
  
positive.code:         HashMap.fromIterables(allClasses, allClasses.map(getInstanceFieldMap));
  
      for (var class_ in allClasses) {
        Set<Class> superclasses;
  
        // Visit accessors in the current class, and see if they override an
        // otherwise private field.
  
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';
  