use symbol literals instead of const ctor in packages

pkg/observe/test/observable_list_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 'package:observe/observe.dart';
 import 'package:unittest/unittest.dart';
 import 'observe_test_utils.dart';
 
 main() {
   // TODO(jmesserly): need all standard List API tests.
 
-  const _LENGTH = const Symbol('length');
-
   StreamSubscription sub;
 
   sharedTearDown() { sub.cancel(); }
 
   group('observe length', () {
 
     ObservableList list;
     List<ChangeRecord> changes;
 
     setUp(() {
       list = toObservable([1, 2, 3]);
       changes = null;
       sub = list.changes.listen((records) {
-        changes = records.where((r) => r.changes(_LENGTH)).toList();
+        changes = records.where((r) => r.changes(#length)).toList();
       });
     });
 
     tearDown(sharedTearDown);
 
     observeTest('add changes length', () {
       list.add(4);
       expect(list, [1, 2, 3, 4]);
       performMicrotaskCheckpoint();
       expectChanges(changes, [_lengthChange]);
@@ skipping 210 matching lines @@
 
       performMicrotaskCheckpoint();
       expectChanges(records, [
         _lengthChange,
         _change(0, removedCount: 6)
       ]);
     });
   });
 }
 
-const _LENGTH = const Symbol('length');
-
-final _lengthChange = new PropertyChangeRecord(_LENGTH);
+final _lengthChange = new PropertyChangeRecord(#length);
 
 _change(index, {removedCount: 0, addedCount: 0}) => new ListChangeRecord(
     index, removedCount: removedCount, addedCount: addedCount);