anchor.diff:  
       test('change data. (T04)', () {
         // given
  -      february['_id'] = 'january';
  +      Map january2 = {'name': 'January2', 'days': 11, 'number': 4, '_id': 'january'};
         return ready.then((_) => months.add(new Map.from(january), 'John Doe'))
   
         // when
  -        .then((_) => months.change('january', new Map.from(february), 'Michael Smith'))
  +        .then((_) => months.change('january', new Map.from(january2), 'Michael Smith'))
           .then((_) => months.data())
           .then((data){
   
         // then
             expect(data['data'].length, equals(1));
             Map strippedData = _stripPrivateFields(data['data'][0]);
  -          expect(strippedData, equals(february));
  +          expect(strippedData, equals(january2));
             expect(data['version'], equals(2));
         }).then((_) => months.diffFromVersion(1))
           .then((dataDiff) {
  
positive.code:         });
      });
  
      test('remove non-existing data. (T08)', () {
        // when
        Future shouldNotThrow =ready.then((_) => months.remove('january', 'Michael Smith'));
  
  
negative.code:           expect(diff['action'], equals('change'));
            expect(diff['_id'], equals('january'));
            Map strippedData = _stripPrivateFields(diff['data']);
            expect(strippedData, equals(january2));
            expect(diff['author'], equals('Michael Smith'));
          });
      });
  
      test('change not existing data. (T05)', () {
        // when
        Future shouldThrow =  ready.then((_) => months.change('january', new Map.from(january), 'Michael Smith'));
  
        //then
          expect(shouldThrow, throws);
  