Improve the style of tests.

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));
+}