anchor.diff:                formatHttpDate(modified.add(const Duration(seconds: 1)))
           };
   
  -        return makeRequest(handler, '/root.txt', headers: headers).then(
  -            (response) {
  +        return makeRequest(handler, '/root.txt', headers: headers)
  +            .then((response) {
             expect(response.statusCode, HttpStatus.NOT_MODIFIED);
             expect(response.contentLength, isNull);
           });
  
positive.code:               formatHttpDate(modified.subtract(const Duration(seconds: 1)))
          };
  
          return makeRequest(handler, '/root.txt', headers: headers).then(
              (response) {
            expect(response.statusCode, HttpStatus.OK);
            expect(response.lastModified, atSameTimeToSecond(modified));
          });
  
negative.code: 
          var headers = {HttpHeaders.IF_MODIFIED_SINCE: formatHttpDate(modified)};
  
          return makeRequest(handler, '/root.txt', headers: headers)
              .then((response) {
            expect(response.statusCode, HttpStatus.NOT_MODIFIED);
            expect(response.contentLength, isNull);
          });
  