use symbol literals instead of const ctor in packages

pkg/polymer_expressions/test/eval_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.
 
 library eval_test;
 
 import 'dart:async';
 
 import 'package:polymer_expressions/eval.dart';
 import 'package:polymer_expressions/filter.dart';
@@ skipping 311 matching lines @@
     });
 
   });
 
 }
 
 class Foo extends Object with ChangeNotifierMixin {
   String _name;
   String get name => _name;
   void set name(String n) {
-    _name = notifyPropertyChange(const Symbol('name'), _name, n);
+    _name = notifyPropertyChange(#name, _name, n);
   }
 
   int age;
   Foo child;
   List<int> items;
 
   Foo({name, this.age, this.child, this.items}) : _name = name;
 
   int x() => age * age;
 }
@@ skipping 47 matching lines @@
   return Future.wait([future, new Future(() {
     expect(passed, true, reason: "Didn't receive a change notification on $s");
   })]);
 }
 
 // Regression test from https://code.google.com/p/dart/issues/detail?id=13459
 class WordElement extends ObservableBase {
   @observable List chars1 = 'abcdefg'.split('');
   List filteredList(List original) => [original[0], original[1]];
 }