Implement HMAC support in lib/crypto.

lib/crypto/sha1.dart

Index: lib/crypto/sha1.dart
diff --git a/lib/crypto/sha1.dart b/lib/crypto/sha1.dart
index 72f57da59bc600a10f64697dfba3c5db9649f82f..e9acfd6aab8a777ef17f5b94fe0d9fc9ae61dacd 100644
--- a/lib/crypto/sha1.dart
+++ b/lib/crypto/sha1.dart
@@ -6,8 +6,6 @@
 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;
@@ -15,6 +13,11 @@ class _SHA1 extends _SHACryptoHashBase implements SHA1 {
     _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;