Make new StreamController be async by default.

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

 // 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 $LIBRARYNAME;
 
 @DocsEditable
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 
   @DomName('Geolocation.getCurrentPosition')
@@ skipping 35 matching lines @@
     }
     if (timeout != null) {
       options['timeout'] = timeout.inMilliseconds;
     }
     if (maximumAge != null) {
       options['maximumAge'] = maximumAge.inMilliseconds;
     }
 
     int watchId;
     var controller;
-    controller = new StreamController<Geoposition>(
+    controller = new StreamController<Geoposition>(sync: true,
       onListen: () {
         assert(watchId == null);
         watchId = $dom_watchPosition(
             (position) {
               controller.add(_ensurePosition(position));
             },
             (error) {
               controller.addError(error);
             },
             options);
@@ skipping 31 matching lines @@
 class _GeopositionWrapper implements Geoposition {
   var _ptr;
   _GeopositionWrapper(this._ptr);
 
   Coordinates get coords => JS('Coordinates', '#.coords', _ptr);
   int get timestamp => JS('int', '#.timestamp', _ptr);
 }
 $endif