Make new StreamController be async by default.

tools/dom/src/PathObserver.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.
 
 part of html;
 
 // This code is inspired by ChangeSummary:
 // https://github.com/rafaelw/ChangeSummary/blob/master/change_summary.js
 // ...which underlies MDV. Since we don't need the functionality of
 // ChangeSummary, we just implement what we need for data bindings.
@@ skipping 54 matching lines @@
    * Observes [path] on [object] for changes. This returns an object that can be
    * used to get the changes and get/set the value at this path.
    * See [PathObserver.values] and [PathObserver.value].
    */
   PathObserver(this.object, String path)
     : path = path, _isValid = _isPathValid(path) {
 
     // TODO(jmesserly): if the path is empty, or the object is! Observable, we
     // can optimize the PathObserver to be more lightweight.
 
-    _values = new StreamController.broadcast(onListen: _observe,
+    _values = new StreamController.broadcast(sync: true,
+                                             onListen: _observe,
                                              onCancel: _unobserve);
 
     if (_isValid) {
       var segments = [];
       for (var segment in path.trim().split('.')) {
         if (segment == '') continue;
         var index = int.parse(segment, onError: (_) {});
         segments.add(index != null ? index : new Symbol(segment));
       }
 
@@ skipping 192 matching lines @@
 
 final _spacesRegExp = new RegExp(r'\s');
 
 bool _isPathValid(String s) {
   s = s.replaceAll(_spacesRegExp, '');
 
   if (s == '') return true;
   if (s[0] == '.') return false;
   return _pathRegExp.hasMatch(s);
 }