anchor.diff:  
       test('map with @serializable class as second key is serialized', () {
         expect(serializedLibrary['collection'],
  -          {DEATHLY_HALLOWS_ISBN: deathlyHallowsMap});
  +          {deathlyHallowsIsbn: deathlyHallowsMap});
       });
     });
   
  
positive.code: import 'dart:convert';
  import 'package:test/test.dart';
  import 'models/author.dart';
  import 'models/book.dart';
  
  const String DEATHLY_HALLOWS_ISBN = '0-545-01022-5';
  
  main() {
    var deathlyHallows = new Book(
  
negative.code:       age: 51,
        books: [deathlyHallows],
        newestBook: deathlyHallows);
    Map serializedJkRowling = AuthorSerializer.toMap(jkRowling);
    Map deathlyHallowsMap = BookSerializer.toMap(deathlyHallows);
    print('J.K. Rowling: $serializedJkRowling');
  
    var library = new Library(collection: {deathlyHallowsIsbn: deathlyHallows});
    var serializedLibrary = LibrarySerializer.toMap(library);
    print('Library: $serializedLibrary');
  
    group('serialization', () {
  