anchor.diff:                contentType: 'foobar');
           }
   
  -        validateServerRequest(e, http.BaseRequest request, List<int> data) {
  +        Future<http.StreamedResponse> validateServerRequest(
  +            e, http.BaseRequest request, List<int> data) {
             return new Future.sync(() {
               var h = e['headers'];
               var r = e['response'];
  
positive.code:       testString(String msg, String expectedBase64) {
          var msgBytes = utf8.encode(msg);
  
          Stream singleByteStream(List<int> msgBytes) {
            var controller = new StreamController();
            for (var byte in msgBytes) {
              controller.add([byte]);
            }
  
negative.code:           // All errors from the [http.Client] propagate through.
            // We use [TestError] to simulate it.
            httpMock.register(expectAsync2((http.BaseRequest request, string) {
              return new Future<http.StreamedResponse>.error(new TestError());
            }), false);
          }
  
  