Make sure to use a fresh hasher instance for all HMAC hashing.

lib/crypto/hmac.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.
 
 class _HMAC implements HMAC {
   _HMAC(Hash this._hash, List<int> this._key) : _message = [];
 
   HMAC update(List<int> data) {
     _message.addAll(data);
     return this;
   }
 
   List<int> digest() {
     var blockSize = _hash.blockSize;
 
     // Hash the key if it is longer than the block size of the hash.
     if (_key.length > blockSize) {
+      _hash = _hash.newInstance();
       _key = _hash.update(_key).digest();
-      _hash = _hash.newInstance();
     }
 
     // Zero-pad the key until its size is equal to the block size of the hash.
     if (_key.length < blockSize) {
       var newKey = new List(blockSize);
       newKey.setRange(0, _key.length, _key);
       for (var i = _key.length; i < blockSize; i++) {
         newKey[i] = 0;
       }
       _key = newKey;
     }
 
     // Compute inner padding.
     var padding = new List(blockSize);
     for (var i = 0; i < blockSize; i++) {
       padding[i] = 0x36 ^ _key[i];
     }
 
     // Inner hash computation.
+    _hash = _hash.newInstance();
     var innerHash = _hash.update(padding).update(_message).digest();
-    _hash = _hash.newInstance();
 
     // Compute outer padding.
     for (var i = 0; i < blockSize; i++) {
       padding[i] = 0x5c ^ _key[i];
     }
 
     // Outer hash computation which is the result.
+    _hash = _hash.newInstance();
     return _hash.update(padding).update(innerHash).digest();
   }
 
   // HMAC internal state.
   Hash _hash;
   List<int> _key;
   List<int> _message;
-}
+}