use symbol literals instead of const ctor in packages

pkg/observe/lib/src/change_notifier.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 observe;
 
 /**
  * Base class implementing [Observable] object that performs its own change
  * notifications, and does not need to be considered by [Observable.dirtyCheck].
  *
@@ skipping 55 matching lines @@
    *
    * 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:
    *
    *     var _myField;
    *     get myField => _myField;
    *     set myField(value) {
-   *       _myField = notifyPropertyChange(
-   *           const Symbol('myField'), _myField, value);
+   *       _myField = notifyPropertyChange(#myField, _myField, value);
    *     }
    */
   notifyPropertyChange(Symbol field, Object oldValue, Object newValue)
       => _notifyPropertyChange(this, field, oldValue, newValue);
 
   void notifyChange(ChangeRecord record) {
     if (!hasObservers) return;
 
     if (_records == null) {
       _records = [];
       runAsync(deliverChanges);
     }
     _records.add(record);
   }
 }