Implement HMAC support in lib/crypto.

tests/lib/src/crypto/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('sha256_long_test_vectors.dart');
 #source('sha256_short_test_vectors.dart');
 
@@ skipping 294 matching lines @@
   }
 }
 
 void main() {
   test();
   testInvalidUse();
   testStandardVectors(sha256_long_inputs, sha256_long_mds);
   testStandardVectors(sha256_short_inputs, sha256_short_mds);
 }