Fix test of contentLength
diff --git a/test/basic_file_test.dart b/test/basic_file_test.dart
index 8348761..b9a1982 100644
--- a/test/basic_file_test.dart
+++ b/test/basic_file_test.dart
@@ -130,7 +130,7 @@
         return makeRequest(handler, '/root.txt', headers: headers)
             .then((response) {
           expect(response.statusCode, HttpStatus.NOT_MODIFIED);
-          expect(response.contentLength, isNull);
+          expect(response.contentLength, 0);
         });
       });
     });
@@ -170,7 +170,7 @@
         return makeRequest(handler, '/root.txt', headers: headers)
             .then((response) {
           expect(response.statusCode, HttpStatus.NOT_MODIFIED);
-          expect(response.contentLength, isNull);
+          expect(response.contentLength, 0);
         });
       });
     });