anchor.diff:    ///
     /// If the constructor is unknown, for instance for native or mirror usage,
     /// `null` is used as key.
  -  Map<ConstructorEntity, Set<Instance>> instantiationMap;
  +  Map<ConstructorEntity?, Set<Instance>>? instantiationMap;
   
     /// Register [type] as the instantiation [kind] using [constructor].
     void addInstantiation(
  -      ConstructorEntity constructor, InterfaceType type, Instantiation kind) {
  -    instantiationMap ??= {};
  -    instantiationMap
  +      ConstructorEntity? constructor, InterfaceType type, Instantiation kind) {
  +    (instantiationMap ??= {})
           .putIfAbsent(constructor, () => Set<Instance>())
           .add(Instance(type, kind));
       switch (kind) {
  
positive.code:       MemberEntity member, ClassEntity type, ClassRelation relation) {
      // TODO(johnniwinther): Use the [member] itself to avoid enqueueing members
      // that are overridden.
      return isInheritedInClass(member.enclosingClass, type, relation);
    }
  
    bool isInheritedInClass(ClassEntity memberHoldingClass, ClassEntity type,
  
negative.code:   /// Registers that [element] has been closurized.
    @override
    void registerClosurizedMember(MemberEntity element) {
      FunctionType type =
          _elementEnvironment.getFunctionType(element as FunctionEntity);
      if (type.containsTypeVariables) {
        _closurizedMembersWithFreeTypeVariables.add(element);
      }
  