Don't assume 32-bit message bit lengths.

lib/src/hash_base.dart

Index: lib/src/hash_base.dart
diff --git a/lib/src/hash_base.dart b/lib/src/hash_base.dart
index c325fe097b89f66ea22b0e86e64b7d584036bc3a..6cdf235ad894273e0917536755a7cd6264834ac3 100644
--- a/lib/src/hash_base.dart
+++ b/lib/src/hash_base.dart
@@ -125,13 +125,17 @@ abstract class HashBase implements Hash {
       _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));
     }
   }
 }