Fix strong mode warnings.

lib/src/hash.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:convert';
 
 import 'digest.dart';
 import 'digest_sink.dart';
 
 /// An interface for cryptographic hash functions.
 ///
 /// Every hash is a converter that takes a list of ints and returns a single
 /// digest. When used in chunked mode, it will only ever add one digest to the
 /// inner [Sink].
-abstract class Hash extends Converter<List<int>, Digest> {
+abstract class Hash
+    extends ChunkedConverter<List<int>, Digest, List<int>, Digest> {
   /// The internal block size of the hash in bytes.
   ///
   /// This is exposed for use by the [Hmac] class, which needs to know the block
   /// size for the [Hash] it uses.
   int get blockSize;
 
   const Hash();
 
   Digest convert(List<int> data) {
     var innerSink = new DigestSink();
     var outerSink = startChunkedConversion(innerSink);
     outerSink.add(data);
     outerSink.close();
     return innerSink.value;
   }
 
   ByteConversionSink startChunkedConversion(Sink<Digest> sink);
 }