Fix bug in pkg:observe (observable_list.remove(Object))

pkg/observe/test/observable_list_test.dart

Index: pkg/observe/test/observable_list_test.dart
diff --git a/pkg/observe/test/observable_list_test.dart b/pkg/observe/test/observable_list_test.dart
index e211243e842975f94636167640c90cb21a23f184..b886f039aafb9c82f53ddb8e38e5a0ea60c54d7b 100644
--- a/pkg/observe/test/observable_list_test.dart
+++ b/pkg/observe/test/observable_list_test.dart
@@ -31,6 +31,15 @@ main() {
       expectChanges(changes, [_lengthChange]);
     });
 
+    test('removeObject', () {
+      list.remove(2);
+      expect(list, orderedEquals([1, 3]));
+
+      deliverChangeRecords();
+      deliverChangeRecords();
+      expectChanges(changes, [_lengthChange]);
+    });
+
     test('removeRange changes length', () {
       list.add(4);
       list.removeRange(1, 3);