Improve the style of tests.

test/utils.dart

Index: test/utils.dart
diff --git a/test/utils.dart b/test/utils.dart
new file mode 100644
index 0000000000000000000000000000000000000000..d65f0ebd21fb66a6c65c4671fa2250d2758c50a0
--- /dev/null
+++ b/test/utils.dart
@@ -0,0 +1,16 @@
+// 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));
+}