Implement HMAC support in lib/crypto.

lib/crypto/sha256.dart

Index: lib/crypto/sha256.dart
diff --git a/lib/crypto/sha256.dart b/lib/crypto/sha256.dart
index f9e836c76900079ffab8a6ca0502c24009b20d36..429b4e6df521e8d94c24acb561d0106044eb69db 100644
--- a/lib/crypto/sha256.dart
+++ b/lib/crypto/sha256.dart
@@ -18,6 +18,11 @@ class _SHA256 extends _SHACryptoHashBase implements SHA256 {
     _h[7] = 0x5be0cd19;
   }
 
+  // Returns a new instance of this CryptoHash.
+  SHA256 newInstance() {
+    return new SHA256();
+  }
+
   // Table of round constants. First 32 bits of the fractional
   // parts of the cube roots of the first 64 prime numbers.
   static final List<int> _K =