Make new StreamController be async by default.

sdk/lib/web_audio/dart2js/web_audio_dart2js.dart

 library dart.dom.web_audio;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_collection-dev';
 import 'dart:html';
 import 'dart:html_common';
 import 'dart:typed_data';
 import 'dart:_js_helper' show Creates, Returns, convertDartClosureToJS;
 import 'dart:_foreign_helper' show JS;
@@ skipping 983 matching lines @@
 
   /**
    * Get a Stream that fires events when AudioProcessingEvents occur.
    * This particular stream is special in that it only allows one listener to a
    * given stream. Converting the returned Stream [asBroadcast] will likely ruin
    * the soft-real-time properties which which these events are fired and can
    * be processed.
    */
   Stream<AudioProcessingEvent> get onAudioProcess {
     if (_eventStream == null) {
-      var controller = new StreamController();
-      var callback = (audioData) { 
+      var controller = new StreamController(sync: true);
+      var callback = (audioData) {
           if (controller.hasListener) {
             // This stream is a strange combination of broadcast and single
             // subscriber streams. We only allow one listener, but if there is
             // no listener, we don't queue up events, we just drop them on the
             // floor.
             controller.add(audioData);
           }
         };
       _setEventListener(callback);
       _eventStream = controller.stream;
@@ skipping 31 matching lines @@
 // 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.
 
 
 @DocsEditable
 @DomName('WaveTable')
 // https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html#WaveTable-section
 @Experimental
 class WaveTable native "WaveTable" {
 }