anchor.diff:    });
   }
   
  +Future _matchesValidValues(Storage storage) {
  +  return storage.getKeys()
  +    .then((List<String> keys) {
  +      expect(keys, unorderedEquals(_validValues.keys));
  +
  +      return Future.forEach(keys, (k) {
  +        return storage.get(k)
  +            .then((dynamic value) {
  +              expect(value, _validValues[k]);
  +            });
  +      });
  +    });
  +}
  +
   const _validValues = const {
     'null': null,
     'string': 'a string',
  
positive.code:           return storage.addAll(_validValues);
          })
          .then((_) {
            return storage.getKeys();
          })
          .then((List<String> keys) {
            expect(keys, unorderedEquals(_validValues.keys));
  
            return Future.forEach(keys, (k) {
              return storage.get(k)
                  .then((dynamic value) {
                    expect(value, _validValues[k]);
                  });
            });
          });
    });
  
  
negative.code:   });
  
    test('store pnp', () {
        return storage.addAll(PNP)
          .then((_) {
            return storage.getKeys();
          })
  