use symbol literals instead of const ctor in packages

pkg/observe/lib/html.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.
 
 // TODO(jmesserly): can we handle this more elegantly?
 // In general, it seems like we want a convenient way to take a Stream plus a
 // getter and convert this into an Observable.
 
 /** Helpers for exposing dart:html as observable data. */
 library observe.html;
@@ skipping 23 matching lines @@
    * Has no effect if the [value] already equals [window.location.hash].
    */
   void set hash(String value) {
     if (value == hash) return;
 
     window.history.pushState(null, '', value);
     _notifyHashChange(null);
   }
 
   void _notifyHashChange(_) {
-    notifyChange(new PropertyChangeRecord(const Symbol('hash')));
+    notifyChange(new PropertyChangeRecord(#hash));
   }
 }
 
 /** Add or remove CSS class [className] based on the [value]. */
 void updateCssClass(Element element, String className, bool value) {
   if (value == true) {
     element.classes.add(className);
   } else {
     element.classes.remove(className);
   }
 }
 
 /** Bind a CSS class to the observable [object] and property [path]. */
 PathObserver bindCssClass(Element element, String className,
     Observable object, String path) {
 
   return new PathObserver(object, path)..bindSync((value) {
     updateCssClass(element, className, value);
   });
 }