Implement HMAC support in lib/crypto.

tests/lib/src/crypto/sha256_test.dart

Index: tests/lib/src/crypto/sha256_test.dart
diff --git a/tests/lib/src/crypto/sha256Test.dart b/tests/lib/src/crypto/sha256_test.dart
similarity index 98%
rename from tests/lib/src/crypto/sha256Test.dart
rename to tests/lib/src/crypto/sha256_test.dart
index ed59d20535cf9cd6d9585029949b47c9b7207786..aa56947423e27c5c31788170c5669862c41bcef7 100644
--- a/tests/lib/src/crypto/sha256Test.dart
+++ b/tests/lib/src/crypto/sha256_test.dart
@@ -292,10 +292,13 @@ 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) {
@@ -308,6 +311,7 @@ void testStandardVectors(inputs, mds) {
 void main() {
   test();
   testInvalidUse();
+  testRepeatedDigest();
   testStandardVectors(sha256_long_inputs, sha256_long_mds);
   testStandardVectors(sha256_short_inputs, sha256_short_mds);
 }