anchor.diff:      });
   
       tearDown(() {
  -      months.dispose();
  +      if (months != null) months.dispose();
       });
   
       test("assign id to data.", () {
  
positive.code:         ];
  
        // when
        handleDiff(diff, months.collection, 'author');
        months.collection.addIndex(['_id']);
  
        // then
  
negative.code: import 'package:clean_data/clean_data.dart';
  import 'dart:async';
  
  class ConnectionMock extends Mock implements Connection {
    ConnectionMock() {
      when(callsTo('send')).alwaysCall((requestFactory) {
        switch (requestFactory().args['action']) {
          case ('get_diff'): return new Future.value({'diff': null});
          case ('get_data'): return new Future.value({'version': 2, 'data': null});
          case ('get_id_prefix'): return new Future.value({'id_prefix': 'prefix'});
          default: return new Future.value(null);
        }
      });
    }
  }
  class IdGeneratorMock extends Mock implements IdGenerator {}
  class CommunicatorMock extends Mock implements Communicator {}
  class FunctionMock extends Mock implements Function {}
  