Implement HMAC support in lib/crypto.

tests/lib/crypto/sha1_test.dart

Index: tests/lib/crypto/sha1_test.dart
diff --git a/tests/lib/crypto/sha1_test.dart b/tests/lib/crypto/sha1_test.dart
index 94245c6e3304cfb10019e56473f227cad604cfba..cb8c97066acaf327ef99e32fa658943f45f14cdb 100644
--- a/tests/lib/crypto/sha1_test.dart
+++ b/tests/lib/crypto/sha1_test.dart
@@ -549,10 +549,13 @@ void test() {
 void testInvalidUse() {
   var sha = new SHA1();
   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 SHA1();
+  var digest = sha.digest();
+  Expect.listEquals(digest, sha.digest());
 }
 
 void testStandardVectors(inputs, mds) {
@@ -565,6 +568,7 @@ void testStandardVectors(inputs, mds) {
 void main() {
   test();
   testInvalidUse();
+  testRepeatedDigest();
   testStandardVectors(sha1_long_inputs, sha1_long_mds);
   testStandardVectors(sha1_short_inputs, sha1_short_mds);
 }