Make new StreamController be async by default.

sdk/lib/mdv_observe_impl/mdv_observe_impl.dart

 // 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.
 
 // This library itself is undocumented and not supported for end use.
 // Because dart:html must use some of this functionality, it has to be available
 // via a dart:* library. The public APIs are reexported via package:mdv_observe.
 // Generally we try to keep this library minimal, with utility types and
 // functions in the package.
 library dart.mdv_observe_impl;
@@ skipping 45 matching lines @@
  */
 typedef ObservableBase = Object with ObservableMixin;
 
 /**
  * Mixin for implementing [Observable] objects.
  *
  * When a field, property, or indexable item is changed, a derived class should
  * call [notifyPropertyChange]. See that method for an example.
  */
 abstract class ObservableMixin implements Observable {
-  StreamController _multiplexController;
+  StreamController _broadcastController;
   List<ChangeRecord> _changes;
 
   Stream<List<ChangeRecord>> get changes {
-    if (_multiplexController == null) {
-      _multiplexController =
-          new StreamController<List<ChangeRecord>>.broadcast();
+    if (_broadcastController == null) {
+      _broadcastController =
+          new StreamController<List<ChangeRecord>>.broadcast(sync: true);
     }
-    return _multiplexController.stream;
+    return _broadcastController.stream;
   }
 
   void _deliverChanges() {
     var changes = _changes;
     _changes = null;
     if (hasObservers && changes != null) {
       // TODO(jmesserly): make "changes" immutable
-      _multiplexController.add(changes);
+      _broadcastController.add(changes);
     }
   }
 
   /**
    * True if this object has any observers, and should call
    * [notifyPropertyChange] for changes.
    */
-  bool get hasObservers => _multiplexController != null &&
-                           _multiplexController.hasListener;
+  bool get hasObservers => _broadcastController != null &&
+                           _broadcastController.hasListener;
 
   /**
    * Notify that the field [name] of this object has been changed.
    *
    * The [oldValue] and [newValue] are also recorded. If the two values are
    * identical, no change will be recorded.
    *
    * For convenience this returns [newValue]. This makes it easy to use in a
    * setter:
    *
@@ skipping 115 matching lines @@
 /** Queues an action to happen during the [deliverChangeRecords] timeslice. */
 void queueChangeRecords(void deliverChanges()) {
   if (_deliverCallbacks == null) {
     _deliverCallbacks = new Queue<Function>();
     runAsync(deliverChangeRecords);
   }
   _deliverCallbacks.add(deliverChanges);
 }
 
 Queue _deliverCallbacks;