Don't assume 32-bit message bit lengths.

lib/src/hash_base.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.
 
 library crypto.hash_base;
 
 import 'dart:math' as math;
 import 'dart:typed_data';
 
 import 'hash.dart';
@@ skipping 107 matching lines @@
   void _finalizeData() {
     _pendingData.add(0x80);
     var contentsLength = _lengthInBytes + 9;
     var chunkSizeInBytes = _chunkSizeInWords * BYTES_PER_WORD;
     var finalizedLength = roundUp(contentsLength, chunkSizeInBytes);
     var zeroPadding = finalizedLength - contentsLength;
     for (var i = 0; i < zeroPadding; i++) {
       _pendingData.add(0);
     }
     var lengthInBits = _lengthInBytes * BITS_PER_BYTE;
-    assert(lengthInBits < math.pow(2, 32));
+    const MAX_UINT64 = 0xFFFFFFFFFFFFFFFF; 
+    if (lengthInBits > MAX_UINT64) {
+      throw new UnsupportedError(
+          "Hash undefined for message bit lengths larger than 64 bits");
+    }
     if (_bigEndianWords) {
-      _pendingData.addAll(_wordToBytes(0));
+      _pendingData.addAll(_wordToBytes((lengthInBits >> 32) & MASK_32));
       _pendingData.addAll(_wordToBytes(lengthInBits & MASK_32));
     } else {
       _pendingData.addAll(_wordToBytes(lengthInBits & MASK_32));
-      _pendingData.addAll(_wordToBytes(0));
+      _pendingData.addAll(_wordToBytes((lengthInBits >> 32) & MASK_32));
     }
   }
 }