Add CryptoUtils class.

tests/lib/crypto/hmac_sha1_test.dart

 // Copyright (c) 2012, 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.
 
 // Library tag to allow the test to run on Dartium.
 #library('hmac_sha1_test');
 
 // TODO(ager): Replace with "dart:crypto" when ready.
 #import("../../../lib/crypto/crypto.dart");
 
 #source('hmac_sha1_test_vectors.dart');
 
-String digestToString(List<int> digest) {
-  var buf = new StringBuffer();
-  for (var part in digest) {
-    buf.add("${(part < 16) ? "0" : ""}${part.toRadixString(16).toLowerCase()}");
-  }
-  return buf.toString();
-}
-
 void testStandardVectors(inputs, keys, macs) {
   for (var i = 0; i < inputs.length; i++) {
     var d = new HMAC(new SHA1(), keys[i]).update(inputs[i]).digest();
-    Expect.isTrue(digestToString(d).startsWith(macs[i]), '$i');
+    Expect.isTrue(CryptoUtils.bytesToHex(d).startsWith(macs[i]), '$i');
   }
 }
 
 void main() {
   testStandardVectors(hmac_sha1_inputs, hmac_sha1_keys, hmac_sha1_macs);
 }