Make new StreamController be async by default.

tools/dom/templates/html/impl/impl_ScriptProcessorNode.darttemplate

 // Copyright (c) 2013, 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 $LIBRARY;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$NATIVESPEC {
   Stream<AudioProcessingEvent> _eventStream;
 
   /**
    * 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;
     }
     return _eventStream;
   }
 
 $if DART2JS
     _setEventListener(callback) {
       JS('void', '#.onaudioprocess = #', this,
           convertDartClosureToJS(callback, 1));
     }
 $endif
 
 $!MEMBERS
 }