Implement HMAC support in lib/crypto.

lib/crypto/sha1.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 SHA1 hasher is used to compute an SHA1 message digest.
 class _SHA1 extends _SHACryptoHashBase implements SHA1 {
   // Construct a SHA1 hasher object.
   _SHA1() : _w = new List(80), super(16, 5) {
-    // 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] = 0x67452301;
     _h[1] = 0xEFCDAB89;
     _h[2] = 0x98BADCFE;
     _h[3] = 0x10325476;
     _h[4] = 0xC3D2E1F0;
   }
 
+  // Returns a new instance of this CryptoHash.
+  SHA1 newInstance() {
+    return new SHA1();
+  }
+
   // Rotate left limiting to unsigned 32-bit values.
   int _rotl32(int val, int shift) {
     var mod_shift = shift & 31;
     return ((val << mod_shift) & _MASK_32) |
         ((val & _MASK_32) >> (32 - mod_shift));
   }
 
   // Compute one iteration of the SHA1 algorithm with a chunk of
   // 16 32-bit pieces.
   void _updateHash(List<int> m) {
@@ skipping 32 matching lines @@
 
     _h[0] = _add32(a, _h[0]);
     _h[1] = _add32(b, _h[1]);
     _h[2] = _add32(c, _h[2]);
     _h[3] = _add32(d, _h[3]);
     _h[4] = _add32(e, _h[4]);
   }
 
   List<int> _w;
 }