Implement HMAC support in lib/crypto.

tests/lib/crypto/sha256_test.dart

Index: tests/lib/crypto/sha256_test.dart
diff --git a/tests/lib/crypto/sha256_test.dart b/tests/lib/crypto/sha256_test.dart
index 5e7ec1e601f13244cc91e0c11db8b8c26c21224a..36d7ecafe60a0982cbd5edcf602e07165275fd3a 100644
--- a/tests/lib/crypto/sha256_test.dart
+++ b/tests/lib/crypto/sha256_test.dart
@@ -293,10 +293,13 @@ void test() {
 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) {
@@ -309,6 +312,7 @@ void testStandardVectors(inputs, mds) {
 void main() {
   test();
   testInvalidUse();
+  testRepeatedDigest();
   testStandardVectors(sha256_long_inputs, sha256_long_mds);
   testStandardVectors(sha256_short_inputs, sha256_short_mds);
 }