anchor.diff:  
       test('map with @serializable class as second key is serialized', () {
         expect(serializedLibrary['collection'],
  -          {DEATHLY_HALLOWS_ISBN: deathlyHallowsMap});
  +          {deathlyHallowsIsbn: deathlyHallowsMap});
       });
     });
   
  
positive.code:       test('map with @serializable class as second key is deserialized', () {
          var lib = new Library.fromJson(serializedLibrary);
          expect(lib.collection, allOf(isNotEmpty, hasLength(1)));
          expect(lib.collection.keys.first, DEATHLY_HALLOWS_ISBN);
          var book = lib.collection[DEATHLY_HALLOWS_ISBN];
          expect(book.id, deathlyHallows.id);
          expect(book.author, deathlyHallows.author);
          expect(book.description, deathlyHallows.description);
  
negative.code: import 'package:test/test.dart';
  import 'models/author.dart';
  import 'models/book.dart';
  
  const String deathlyHallowsIsbn = '0-545-01022-5';
  
  main() {
    var deathlyHallows = new Book(
  