Add StreamChannelTransformer.

lib/src/isolate_channel.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:async';
 import 'dart:isolate';
 
 import '../stream_channel.dart';
 
 /// A [StreamChannel] that communicates over a [ReceivePort]/[SendPort] pair,
@@ skipping 110 matching lines @@
     }
 
     return done;
   }
 
   Future addStream(Stream<T> stream) {
     if (_closed) throw new StateError("Cannot add stream after closing.");
     if (_inAddStream) {
       throw new StateError("Cannot add stream while adding stream.");
     }
-    if (_isDone) return;
+    if (_isDone) return new Future.value();
 
     _inAddStream = true;
     var completer = new Completer.sync();
     stream.listen(_add,
         onError: (error, stackTrace) {
           _close(error, stackTrace);
           completer.complete();
         },
         onDone: completer.complete,
         cancelOnError: true);
     return completer.future.then((_) {
       _inAddStream = false;
     });
   }
 }