Implement HMAC support in lib/crypto.

tests/lib/src/crypto/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.
 
 // TODO(ager): Replace with "dart:crypto" when ready.
 #import("../../../../lib/crypto/crypto.dart");
 
 #source('sha1_long_test_vectors.dart');
 #source('sha1_short_test_vectors.dart');
 
@@ skipping 549 matching lines @@
     Expect.equals(mds[i], digestToString(d), '$i');
   }
 }
 
 void main() {
   test();
   testInvalidUse();
   testStandardVectors(sha1_long_inputs, sha1_long_mds);
   testStandardVectors(sha1_short_inputs, sha1_short_mds);
 }