use symbol literals instead of const ctor in packages

pkg/observe/lib/src/observable_list.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;
 
 /**
  * Represents an observable list of model values. If any items are added,
  * removed, or replaced, then observers that are listening to [changes]
  * will be notified.
@@ skipping 207 matching lines @@
   // list modifications.
   // One simple idea: we can simply update the index map as we do the operations
   // to the list, then produce the records at the end.
   void _summarizeRecords() {
     int oldLength = length;
     for (var r in _listRecords) {
       oldLength += r.removedCount - r.addedCount;
     }
 
     if (length != oldLength) {
-      notifyPropertyChange(const Symbol('length'), oldLength, length);
+      notifyPropertyChange(#length, oldLength, length);
     }
 
     if (_listRecords.length == 1) {
       notifyChange(_listRecords[0]);
       _listRecords = null;
       return;
     }
 
     var items = [];
     for (int i = 0; i < oldLength; i++) items.add(i);
@@ skipping 32 matching lines @@
 
       if (added > 0 || removed > 0) {
         notifyChange(new ListChangeRecord(startIndex, addedCount: added,
             removedCount: removed));
       }
 
       offset += removed - added;
     }
   }
 }