Implement HMAC support in lib/crypto.

tests/lib/src/crypto/hmac_sha256_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.
+
+// TODO(ager): Replace with "dart:crypto" when ready.
+#import("../../../../lib/crypto/crypto.dart");
+
+#source('hmac_sha256_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 SHA256(), keys[i]).update(inputs[i]).digest();
+    Expect.isTrue(digestToString(d).startsWith(macs[i]), '$i');
+  }
+}
+
+void main() {
+  testStandardVectors(hmac_sha256_inputs, hmac_sha256_keys, hmac_sha256_macs);
+}