use symbol literals instead of const ctor in packages

pkg/polymer_expressions/test/syntax_test.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.
 
 import 'dart:async';
 import 'dart:html';
 
 import 'package:mdv/mdv.dart' as mdv;
 import 'package:logging/logging.dart';
 import 'package:observe/observe.dart';
@@ skipping 64 matching lines @@
           expect(records.length, 1);
           expect(records.first.message,
               contains('Error evaluating expression'));
           expect(records.first.message, contains('foo'));
         });
       });
     });
   });
 }
 
-class Person extends Object with ChangeNotifierMixin {
-  static const _FIRST_NAME = const Symbol('firstName');
-  static const _LAST_NAME = const Symbol('lastName');
-  static const _ITEMS = const Symbol('items');
-  static const _GET_FULL_NAME = const Symbol('getFullName');
-
+class Person extends ChangeNotifierBase {
   String _firstName;
   String _lastName;
   List<String> _items;
 
   Person(this._firstName, this._lastName, this._items);
 
   String get firstName => _firstName;
 
   void set firstName(String value) {
     _firstName = value;
-    notifyChange(new PropertyChangeRecord(_FIRST_NAME));
+    notifyChange(new PropertyChangeRecord(#firstName));
   }
 
   String get lastName => _lastName;
 
   void set lastName(String value) {
     _lastName = value;
-    notifyChange(new PropertyChangeRecord(_LAST_NAME));
+    notifyChange(new PropertyChangeRecord(#lastName));
   }
 
   String getFullName() => '$_firstName $_lastName';
 
   List<String> get items => _items;
 
   void set items(List<String> value) {
     _items = value;
-    notifyChange(new PropertyChangeRecord(_ITEMS));
+    notifyChange(new PropertyChangeRecord(#items));
   }
 
   String toString() => "Person(firstName: $_firstName, lastName: $_lastName)";
 
 }