Implement HMAC support in lib/crypto.

lib/crypto/sha256.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.
 
 // The SHA256 hasher is used to compute an SHA256 message digest.
-class _SHA256 extends _SHACryptoHashBase implements SHA256 {
+class _SHA256 extends _SHAHashBase implements SHA256 {
   // Construct a SHA256 hasher object.
   _SHA256() : _w = new List(64), super(16, 8) {
     // Initial value of the hash parts. First 32 bits of the fractional parts
     // of the square roots of the first 8 prime numbers.
     _h[0] = 0x6a09e667;
     _h[1] = 0xbb67ae85;
     _h[2] = 0x3c6ef372;
     _h[3] = 0xa54ff53a;
     _h[4] = 0x510e527f;
     _h[5] = 0x9b05688c;
     _h[6] = 0x1f83d9ab;
     _h[7] = 0x5be0cd19;
   }
 
+  // Returns a new instance of this Hash.
+  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 =
       const [ 0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b,
               0x59f111f1, 0x923f82a4, 0xab1c5ed5, 0xd807aa98, 0x12835b01,
               0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7,
               0xc19bf174, 0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc,
               0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da, 0x983e5152,
               0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147,
               0x06ca6351, 0x14292967, 0x27b70a85, 0x2e1b2138, 0x4d2c6dfc,
@@ skipping 58 matching lines @@
     _h[2] = _add32(c, _h[2]);
     _h[3] = _add32(d, _h[3]);
     _h[4] = _add32(e, _h[4]);
     _h[5] = _add32(f, _h[5]);
     _h[6] = _add32(g, _h[6]);
     _h[7] = _add32(h, _h[7]);
   }
 
   List<int> _w;
 }