Make new StreamController be async by default.

samples/swarm/swarm_ui_lib/touch/Scroller.dart

 // Copyright (c) 2011, 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 touch;
 
 /**
  * Implementation of a custom scrolling behavior.
  * This behavior overrides native scrolling for an area. This area can be a
  * single defined part of a page, the entire page, or several different parts
@@ skipping 228 matching lines @@
     // TODO(jacobr): this assert fires asynchronously which could be confusing.
     if (_scrollTechnique == ScrollerScrollTechnique.RELATIVE_POSITIONING) {
       assert(_element.getComputedStyle().position != "static");
     }
 
     _initLayer();
   }
 
   Stream<Event> get onScrollerStart {
     if (_onScrollerStart == null) {
-      _onScrollerStart = new StreamController<Event>();
+      _onScrollerStart = new StreamController<Event>(sync: true);
       _onScrollerStartStream = _onScrollerStart.stream.asBroadcastStream();
     }
     return _onScrollerStartStream;
   }
 
   Stream<Event> get onScrollerEnd {
     if (_onScrollerEnd == null) {
-      _onScrollerEnd = new StreamController<Event>();
+      _onScrollerEnd = new StreamController<Event>(sync: true);
       _onScrollerEndStream = _onScrollerEnd.stream.asBroadcastStream();
     }
     return _onScrollerEndStream;
   }
 
   Stream<Event> get onScrollerDragEnd {
     if (_onScrollerDragEnd == null) {
-      _onScrollerDragEnd = new StreamController<Event>();
+      _onScrollerDragEnd = new StreamController<Event>(sync: true);
       _onScrollerDragEndStream = _onScrollerDragEnd.stream.asBroadcastStream();
     }
     return _onScrollerDragEndStream;
   }
 
   Stream<Event> get onContentMoved {
     if (_onContentMoved == null) {
-      _onContentMoved = new StreamController<Event>();
+      _onContentMoved = new StreamController<Event>(sync: true);
       _onContentMovedStream = _onContentMoved.stream.asBroadcastStream();
     }
     return _onContentMovedStream;
   }
 
   Stream<Event> get onDecelStart {
     if (_onDecelStart == null) {
-      _onDecelStart = new StreamController<Event>();
+      _onDecelStart = new StreamController<Event>(sync: true);
       _onDecelStartStream = _onDecelStart.stream.asBroadcastStream();
     }
     return _onDecelStartStream;
   }
 
 
   /**
    * Add a scroll listener. This allows other classes to subscribe to scroll
    * notifications from this scroller.
    */
@@ skipping 402 matching lines @@
   static const SCROLLER_END = "scroller:scroll_end";
   static const DRAG_END = "scroller:drag_end";
   static const CONTENT_MOVED = "scroller:content_moved";
   static const DECEL_START = "scroller:decel_start";
 }
 
 class ScrollerScrollTechnique {
   static const TRANSFORM_3D = 1;
   static const RELATIVE_POSITIONING = 2;
 }