anchor.diff:              .get(0)
               .then((value) => expect(value, '0', reason: 'value is loaded'))
               .then((value) => expect(counter, 1, reason: 'loaded once'))
  -            .then((_) => offset = new Duration(seconds: 20))
  +            .then((_) => offset = const Duration(seconds: 20))
               .then((_) => cache.get(0))
               .then((value) => expect(value, '0', reason: 'value is still present'))
               .then((value) => expect(counter, 1, reason: 'still loaded once'))
  -            .then((_) => offset = new Duration(seconds: 21))
  +            .then((_) => offset = const Duration(seconds: 21))
               .then((_) => cache.get(0))
               .then((value) => expect(value, '0', reason: 'value is reloaded'))
               .then((value) => expect(counter, 2, reason: 'value loaded again'));
  
positive.code:         var cache = newAccessExpireCache(immediateLoader);
          return cache
              .set(1, '1')
              .then((_) => offset = new Duration(seconds: 120))
              .then((_) => cache.size())
              .then((size) => expect(size, 1, reason: 'cache has not been reaped'))
              .then((_) => cache.reap())
  
negative.code:         return cache
              .get(1)
              .then((value) => expect(value, '1', reason: 'value is loaded'))
              .then((_) => offset = const Duration(seconds: 20))
              .then((_) => cache.getIfPresent(1))
              .then((value) => expect(value, '1', reason: 'value is still present'))
              .then((_) => offset = const Duration(seconds: 21))
              .then((_) => cache.getIfPresent(1))
              .then((value) => expect(value, isNull, reason: 'value has expried'))
              .then((_) => cache.size())
  