Transaction: a nullable return type becomes not nullable in the method's implementation

[Mabsten]

Step to reproduce the issue:

• add this simple method to the TaskDao class in the official example

  @transaction
   Future<Task?> test(int id) {
     return findTaskById(id);
   }

• run flutter pub run build_runner build

Generated code:

  @override
  Future<Task> test(int id) async {
    if (database is sqflite.Transaction) {
      return super.test(id);
    } else {
      return (database as sqflite.Database)
          .transaction<Task>((transaction) async {
        final transactionDatabase = _$FlutterDatabase(changeListener)
          ..database = transaction;
        return transactionDatabase.taskDao.test(id);
      });
    }
  }

This code doesn't compile because the method is declared with a not nullable return type, and also sqflite.Database.transaction<Task>(..) (in the else branch) is invoked with a not nullable parametric type

[StarkShang]

This append is 1.2.0 again even with annotation on top of database.dart file:

//@Dart=2.12