Make HMAC a Converter.

test/utils.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:math' as math;
 import 'dart:io';
 
 import 'package:crypto/crypto.dart';
 import 'package:test/test.dart';
 
 /// Asserts that an HMAC using [hash] returns [mac] for [input] and [key].
 void expectHmacEquals(Hash hash, List<int> input, List<int> key, String mac) {
   var hmac = new HMAC(hash, key);
-  hmac.add(input);
-  expect(CryptoUtils.bytesToHex(hmac.close()), startsWith(mac));
+  expect(hmac.convert(input).toString(), startsWith(mac));
 }