anchor.diff:  
       tearDown(() {
         mongodb.close();
  -
       });
   
       test('get data. (T01)', () {
         // when
         return ready.then((_) {
  +
           // then
           return months.data().then((Map data) {
             expect(data['data'], equals([]));
  
positive.code: 
      test('add data with same _id. (T03)', () {
  // given
        february['_id'] = 'january';
        Future shouldThrow = ready.then((_) => months.add(new Map.from(january), 'John Doe'))
  
        // when
        .then((_) => months.add(new Map.from(february), 'John Doe'));
  
        // then
  
        expect(shouldThrow, throws);
  
      });
  
  
      test('change data. (T04)', () {
        // given
  
negative.code:   });
  }
  
  Map _stripPrivateFields(Map<String, dynamic> data){
    Map newData = {};
    data.forEach((key,value) {
      if (!key.startsWith('__')) newData[key] = value;
    });
    return newData;
  }
  
  List _stripPrivateFieldsList(List<Map<String, dynamic>> data){
    List newData = [];
    data.forEach((oldMap) {
      newData.add(_stripPrivateFields(oldMap));
    });
    return newData;
  }
  
  void main() {
    group('MongoProvider', () {
      MongoProvider months;
      Future ready;
      MongoDatabase mongodb;
      Map january, february, march, april, may, june, july,
          august, september, october, november, december;
  
       setUp(() {
        january = {'name': 'January', 'days': 31, 'number': 1, '_id': 'january'};
        february = {'name': 'February', 'days': 28, 'number': 2, '_id': 'february'};
        march =  {'name': 'March', 'days': 31, 'number': 3, '_id': 'march'};
  