anchor.diff:        age: 51,
         books: [deathlyHallows],
         newestBook: deathlyHallows);
  -  Map serializedJkRowling = JSON.decode(JSON.encode(jkRowling.toJson()));
  -  Map deathlyHallowsMap = JSON.decode(JSON.encode(serializedDeathlyHallows));
  +  Map serializedJkRowling = AuthorSerializer.toMap(jkRowling);
  +  Map deathlyHallowsMap = BookSerializer.toMap(deathlyHallows);
     print('J.K. Rowling: $serializedJkRowling');
   
  -  var library = new Library(collection: {DEATHLY_HALLOWS_ISBN: deathlyHallows});
  -  var serializedLibrary = JSON.decode(JSON.encode(library.toJson()));
  +  var library = new Library(collection: {deathlyHallowsIsbn: deathlyHallows});
  +  var serializedLibrary = LibrarySerializer.toMap(library);
     print('Library: $serializedLibrary');
   
     group('serialization', () {
  
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: 
      test('map with @serializable class as second key is serialized', () {
        expect(serializedLibrary['collection'],
            {deathlyHallowsIsbn: deathlyHallowsMap});
      });
    });
  
  