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 274 matching lines @@
 
   for (var i = 0; i < expected_values.length; i++) {
     var d = new SHA256().update(createTestArr(i)).digest();
     Expect.equals(expected_values[i], digestToString(d), '$i');
   }
 }
 
 void testInvalidUse() {
   var sha = new SHA256();
   sha.digest();
-  Expect.throws(sha.digest,
-                (e) => e is CryptoHashException);
-  Expect.throws(() => sha.update([0]),
-                (e) => e is CryptoHashException);
+  Expect.throws(() => sha.update([0]), (e) => e is HashException);
+}
+
+void testRepeatedDigest() {
+  var sha = new SHA256();
+  var digest = sha.digest();
+  Expect.listEquals(digest, sha.digest());
 }
 
 void testStandardVectors(inputs, mds) {
   for (var i = 0; i < inputs.length; i++) {
     var d = new SHA256().update(inputs[i]).digest();
     Expect.equals(mds[i], digestToString(d), '$i');
   }
 }
 
 void main() {
   test();
   testInvalidUse();
+  testRepeatedDigest();
   testStandardVectors(sha256_long_inputs, sha256_long_mds);
   testStandardVectors(sha256_short_inputs, sha256_short_mds);
 }