use symbol literals instead of const ctor in packages

pkg/observe/lib/src/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;
 
 // 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 114 matching lines @@
       if (identical(_values[i + 1], newValue)) {
         _observePath(start, i);
         return;
       }
       _values[i + 1] = newValue;
       changed = true;
     }
 
     _observePath(start);
     if (changed) {
-      notifyChange(new PropertyChangeRecord(const Symbol('value')));
+      notifyChange(new PropertyChangeRecord(#value));
     }
   }
 
   void _observePath([int start = 0, int end]) {
     if (end == null) end = _segments.length;
 
     for (int i = start; i < end; i++) {
       if (_subs[i] != null) _subs[i].cancel();
       _observeIndex(i);
     }
@@ skipping 92 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);
 }