Add StreamChannel.withCloseGuarantee.

lib/src/json_document_transformer.dart

 // Copyright (c) 2016, 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 'package:async/async.dart';
 
 import '../stream_channel.dart';
 import 'stream_channel_transformer.dart';
@@ skipping 22 matching lines @@
   JsonDocumentTransformer({reviver(key, value), toEncodable(object)})
       : _codec = new JsonCodec(reviver: reviver, toEncodable: toEncodable);
 
   JsonDocumentTransformer._(this._codec);
 
   StreamChannel bind(StreamChannel<String> channel) {
     var stream = channel.stream.map(_codec.decode);
     var sink = new StreamSinkTransformer.fromHandlers(handleData: (data, sink) {
       sink.add(_codec.encode(data));
     }).bind(channel.sink);
-    return new StreamChannel(stream, sink);
+    return new StreamChannel.withCloseGuarantee(stream, sink);
   }
 }