anchor.diff:  /// Here, we extended the base service class. This allows to only expose
   /// specific methods, and also allows more freedom over things such as validation.
   class UserService extends Service {
  -  MongoTypedService<User> _inner;
  +  TypedService<User> _inner;
   
     UserService(DbCollection collection) : super() {
  -    _inner = new MongoTypedService<User>(collection);
  +    _inner = new TypedService<User>(new MongoService(collection));
     }
   
     @override
  
positive.code: import 'package:crypto/crypto.dart' show sha256;
  import 'package:mongo_dart/mongo_dart.dart';
  import '../models/user.dart';
  export '../models/user.dart';
  
  configureServer(Db db) {
  
negative.code:     app.container.singleton(service.inner);
  
      service.beforeCreated
        ..listen(validateEvent(CREATE_USER))
        ..listen((e) {
          e.data['password'] = hashPassword(e.data['password']);
        });
  