use symbol literals instead of const ctor in packages

pkg/observe/test/observable_map_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 Map API tests.
 
   StreamSubscription sub;
 
   sharedTearDown() {
     if (sub != null) {
       sub.cancel();
       sub = null;
     }
   }
 
   group('observe length', () {
     ObservableMap map;
     List<ChangeRecord> changes;
 
     setUp(() {
       map = toObservable({'a': 1, 'b': 2, 'c': 3});
       changes = null;
       sub = map.changes.listen((records) {
-        changes = records.where((r) => r.changes(_LENGTH)).toList();
+        changes = records.where((r) => r.changes(#length)).toList();
       });
     });
 
     tearDown(sharedTearDown);
 
     observeTest('add item changes length', () {
       map['d'] = 4;
       expect(map, {'a': 1, 'b': 2, 'c': 3, 'd': 4});
       performMicrotaskCheckpoint();
       expectChanges(changes, [_lengthChange]);
@@ skipping 203 matching lines @@
       performMicrotaskCheckpoint();
       expectChanges(records, [
         _change('a', isRemove: true),
         _change('b', isRemove: true),
         _lengthChange,
       ]);
     });
   });
 }
 
-
-const _LENGTH = const Symbol('length');
-
-final _lengthChange = new PropertyChangeRecord(_LENGTH);
+final _lengthChange = new PropertyChangeRecord(#length);
 
 _change(key, {isInsert: false, isRemove: false}) =>
     new MapChangeRecord(key, isInsert: isInsert, isRemove: isRemove);