Make new StreamController be async by default.

sdk/lib/isolate/isolate_stream.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.
 
 part of dart.isolate;
 
 /**
  * The initial [IsolateStream] available by default for this isolate. This
  * [IsolateStream] is created automatically and it is commonly used to establish
  * the first communication between isolates (see [streamSpawnFunction] and
@@ skipping 20 matching lines @@
 
 /**
  * [IsolateStream]s, together with [IsolateSink]s, are the only means of
  * communication between isolates. Each IsolateStream has a corresponding
  * [IsolateSink]. Any message written into that sink will be delivered to
  * the stream and then dispatched to the stream's subscribers.
  */
 class IsolateStream extends Stream<dynamic> {
   bool _isClosed = false;
   final ReceivePort _port;
-  StreamController _controller = new StreamController();
+  StreamController _controller = new StreamController(sync: true);
 
   IsolateStream._fromOriginalReceivePort(this._port) {
     _port.receive((message, replyTo) {
       assert(replyTo == null);
       _add(message);
     });
   }
 
   IsolateStream._fromOriginalReceivePortOneShot(this._port) {
     _port.receive((message, replyTo) {
@@ skipping 89 matching lines @@
  * default stream.
  *
  * The optional [unhandledExceptionCallback] argument is invoked whenever an
  * exception inside the isolate is unhandled. It can be seen as a big
  * `try/catch` around everything that is executed inside the isolate. The
  * callback should return `true` when it was able to handled the exception.
  */
 external IsolateSink streamSpawnFunction(
     void topLevelFunction(),
     [bool unhandledExceptionCallback(IsolateUnhandledException e)]);