fix observing superclass fields, issue 13255

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(watch: true); });
+  group('WatcherModel', () => _observeTests((x) => new WatcherModel(x)));
 
-  group('ObservableBox', () { _observeTests(); });
+  group('ObservableBox', () => _observeTests((x) => new ObservableBox(x)));
+
+  group('ModelSubclass', () => _observeTests((x) => new ModelSubclass(x)));
 
   group('dirtyCheck loops can be debugged', () {
     var messages;
     var subscription;
     setUp(() {
       messages = [];
       subscription = Logger.root.onRecord.listen((record) {
         messages.add(record.message);
       });
     });
@@ skipping 15 matching lines @@
 
       expect(messages[0], contains('Possible loop'));
       expect(messages[1], contains('index 0'));
       expect(messages[1], contains('object: $x'));
 
       sub.cancel();
     });
   });
 }
 
-void _observeTests({bool watch: false}) {
-  final createModel = watch ? (x) => new WatcherModel(x)
-      : (x) => new ObservableBox(x);
+void _observeTests(createModel(x)) {
+  final watch = createModel(null) is! ChangeNotifierMixin;
 
   // Track the subscriptions so we can clean them up in tearDown.
   List subs;
 
   int initialObservers;
   setUp(() {
     initialObservers = dirty_check.allObservablesCount;
     subs = [];
 
     if (watch) runAsync(Observable.dirtyCheck);
@@ skipping 172 matching lines @@
 _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);
+}