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

pkg/observe/test/observe_test_utils.dart

Index: pkg/observe/test/observe_test_utils.dart
diff --git a/pkg/observe/test/observe_test_utils.dart b/pkg/observe/test/observe_test_utils.dart
index 2025e1b87addf546708cd4727d9ecfcb050064da..af447dcff126bfc7b79dae0b7d1632551218cd06 100644
--- a/pkg/observe/test/observe_test_utils.dart
+++ b/pkg/observe/test/observe_test_utils.dart
@@ -17,7 +17,7 @@ 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();