[observe] add isEmpty/isNotEmpty to ObservableList reflection

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() {
@@ skipping 235 matching lines @@
         _change(1, addedCount: 1),
         _change(4, removed: [1])
       ]);
     });
 
     observeTest('clear', () {
       list.clear();
       expect(list, []);
 
       performMicrotaskCheckpoint();
-      expectChanges(propRecords, [_lengthChange(6, 0)]);
+      expectChanges(propRecords, [
+          _lengthChange(6, 0),
+          new PropertyChangeRecord(list, #isEmpty, false, true),
+          new PropertyChangeRecord(list, #isNotEmpty, true, false),
+      ]);
       expectChanges(listRecords, [_change(0, removed: [1, 2, 3, 1, 3, 4])]);
     });
   });
 }
 
 ObservableList list;
 
 _lengthChange(int oldValue, int newValue) =>
     new PropertyChangeRecord(list, #length, oldValue, newValue);
 
 _change(index, {removed: const [], addedCount: 0}) => new ListChangeRecord(
     list, index, removed: removed, addedCount: addedCount);