use symbol literals instead of const ctor in packages

pkg/observe/test/observe_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:logging/logging.dart';
 import 'package:observe/observe.dart';
 import 'package:observe/src/dirty_check.dart' as dirty_check;
 import 'package:unittest/unittest.dart';
 import 'observe_test_utils.dart';
 
 // Note: this ensures we run the dartanalyzer on the @observe package.
 // @static-clean
 
-const _VALUE = const Symbol('value');
-
 void main() {
   // Note: to test the basic Observable system, we use ObservableBox due to its
   // simplicity. We also test a variant that is based on dirty-checking.
 
   observeTest('no observers at the start', () {
     expect(dirty_check.allObservablesCount, 0);
   });
 
   group('WatcherModel', () => _observeTests((x) => new WatcherModel(x)));
 
@@ skipping 180 matching lines @@
     var records = null;
     subs.add(t.changes.listen((r) { records = r; }));
     t.value = 42;
 
     performMicrotaskCheckpoint();
     expectChanges(records, _changedValue(1));
 
     // Verify that mutation operations on the list fail:
 
     expect(() {
-      records[0] = new PropertyChangeRecord(_VALUE);
+      records[0] = new PropertyChangeRecord(#value);
     }, throwsUnsupportedError);
 
     expect(() { records.clear(); }, throwsUnsupportedError);
 
     expect(() { records.length = 0; }, throwsUnsupportedError);
   });
 
   observeTest('notifyChange', () {
     var t = createModel(123);
     var records = [];
     subs.add(t.changes.listen((r) { records.addAll(r); }));
-    t.notifyChange(new PropertyChangeRecord(_VALUE));
+    t.notifyChange(new PropertyChangeRecord(#value));
 
     performMicrotaskCheckpoint();
     expectChanges(records, _changedValue(1));
     expect(t.value, 123, reason: 'value did not actually change.');
   });
 
   observeTest('notifyPropertyChange', () {
     var t = createModel(123);
     var records = null;
     subs.add(t.changes.listen((r) { records = r; }));
-    expect(t.notifyPropertyChange(_VALUE, t.value, 42), 42,
+    expect(t.notifyPropertyChange(#value, t.value, 42), 42,
         reason: 'notifyPropertyChange returns newValue');
 
     performMicrotaskCheckpoint();
     expectChanges(records, _changedValue(1));
     expect(t.value, 123, reason: 'value did not actually change.');
   });
 }
 
-_changedValue(len) => new List.filled(len, new PropertyChangeRecord(_VALUE));
+_changedValue(len) => new List.filled(len, new PropertyChangeRecord(#value));
 
 // A test model based on dirty checking.
 class WatcherModel<T> extends ObservableBase {
   @observable T value;
 
   WatcherModel([T initialValue]) : value = initialValue;
 
   String toString() => '#<$runtimeType value: $value>';
 }
 
 class ModelSubclass<T> extends WatcherModel<T> {
   ModelSubclass([T initialValue]) : super(initialValue);
 }