anchor.diff:  
       Cache<int, String> newUpdateExpireCache(Loader<int, String> loader) {
         return new Cache.expiry(
  -          loader: loader, clock: offsetClock, updateExpiry: new Duration(seconds: 20));
  +          loader: loader, clock: offsetClock, updateExpiry: const Duration(seconds: 20));
       }
   
       Cache<int, String> newAccessExpireCache(Loader<int, String> loader) {
         return new Cache.expiry(
  -          loader: loader, clock: offsetClock, accessExpiry: new Duration(seconds: 20));
  +          loader: loader, clock: offsetClock, accessExpiry: const Duration(seconds: 20));
       }
   
       statelessCacheTests(newUpdateExpireCache);
  
positive.code:         var cache = newAccessExpireCache(immediateLoader);
          return cache
              .set(1, '2')
              .then((_) => offset = new Duration(seconds: 20))
              .then((_) => cache.getIfPresent(1))
              .then((value) => expect(value, '2', reason: 'value is set'))
              .then((_) => cache.set(1, '3'))
              .then((_) => offset = new Duration(seconds: 40))
              .then((_) => cache.getIfPresent(1))
              .then((value) => expect(value, '3', reason: 'value is updated'))
              .then((_) => offset = new Duration(seconds: 61))
              .then((_) => cache.getIfPresent(1))
              .then((value) => expect(value, isNull, reason: 'value has expried'));
        });
  
negative.code:             .get(0)
              .then((value) => expect(value, '0', reason: 'value is loaded'))
              .then((value) => expect(counter, 1, reason: 'loaded once'))
              .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 = 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'));
  