introduce ObservableList.listChanges to keep list changes separate from property changes

pkg/observe/test/observe_test_utils.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 observe.test.observe_test_utils;
 
 import 'package:observe/observe.dart';
 
 import 'package:unittest/unittest.dart';
 
 import 'package:observe/src/microtask.dart';
 export 'package:observe/src/microtask.dart';
 
 // TODO(jmesserly): use matchers when we have a way to compare ChangeRecords.
 // For now just use the toString.
 expectChanges(actual, expected, {reason}) =>
     expect('$actual', '$expected', reason: reason);
 
 List getListChangeRecords(List changes, int index) => changes
-    .where((c) => c is ListChangeRecord && c.indexChanged(index)).toList();
+    .where((c) => c.indexChanged(index)).toList();
 
 List getPropertyChangeRecords(List changes, Symbol property) => changes
     .where((c) => c is PropertyChangeRecord && c.name == property).toList();
 
 /**
  * This is a special kind of unit [test], that supports
  * calling [performMicrotaskCheckpoint] during the test to pump events
  * that original from observable objects.
  */
 observeTest(name, testCase) => test(name, wrapMicrotask(testCase));
 
 /** The [solo_test] version of [observeTest]. */
 solo_observeTest(name, testCase) => solo_test(name, wrapMicrotask(testCase));