use symbol literals instead of const ctor in packages

pkg/observe/lib/src/list_path_observer.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;
 
-const _VALUE = const Symbol('value');
-
 // Inspired by ArrayReduction at:
 // https://raw.github.com/rafaelw/ChangeSummary/master/util/array_reduction.js
 // The main difference is we support anything on the rich Dart Iterable API.
 
 /**
  * Observes a path starting from each item in the list.
  */
 class ListPathObserver<E, P> extends ChangeNotifierBase {
   final ObservableList<E> list;
   final String _itemPath;
@@ skipping 23 matching lines @@
 
   void dispose() {
     if (_sub != null) _sub.cancel();
     _subs.forEach((s) => s.cancel());
     _subs.clear();
   }
 
   void _reduce() {
     _scheduled = false;
     _value = _observers.map((o) => o.value);
-    notifyChange(new PropertyChangeRecord(_VALUE));
+    notifyChange(new PropertyChangeRecord(#value));
   }
 
   void _scheduleReduce(_) {
     if (_scheduled) return;
     _scheduled = true;
     runAsync(_reduce);
   }
 
   void _observeItems(int lengthAdjust) {
     if (lengthAdjust > 0) {
       for (int i = 0; i < lengthAdjust; i++) {
         int len = _observers.length;
         var pathObs = new PathObserver(list, '$len.$_itemPath');
         _subs.add(pathObs.changes.listen(_scheduleReduce));
         _observers.add(pathObs);
       }
     } else if (lengthAdjust < 0) {
       for (int i = 0; i < -lengthAdjust; i++) {
         _subs.removeLast().cancel();
       }
       int len = _observers.length;
       _observers.removeRange(len + lengthAdjust, len);
     }
   }
 }