Fix list_path_observer, temporarily add a test for it, but we'll likely remove

pkg/observe/test/list_path_observer_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() => dirtyCheckZone().run(_runTests);
+
+_runTests() {
+  var list;
+  var obs;
+  var o1, o2, o3;
+  var sub;
+  int changes;
+
+  setUp(() {
+    list = toObservable([
+      o1 = new TestModel()..a = (new TestModel()..b = 1),
+      o2 = new TestModel()..a = (new TestModel()..b = 2),
+      o3 = new TestModel()..a = (new TestModel()..b = 3)]);
+    obs = new ListPathObserver(list, 'a.b');
+    changes = 0;
+    sub = obs.changes.listen((e) { changes++; });
+  });
+
+  tearDown(() {
+    sub.cancel();
+    list = obs = o1 = o2 = o3 = null;
+  });
+
+  test('list path observer noticed length changes', () {
+    expect(o2.a.b, 2);
+    expect(list[1].a.b, 2);
+    return _nextMicrotask(null).then((_) {
+      expect(changes, 0);
+      list.removeAt(1);
+    }).then(_nextMicrotask).then((_) {
+      expect(changes, 1);
+      expect(list[1].a.b, 3);
+    });
+  });
+
+  test('list path observer delivers deep change', () {
+    expect(o2.a.b, 2);
+    expect(list[1].a.b, 2);
+    int changes = 0;
+    obs.changes.listen((e) { changes++; });
+    return _nextMicrotask(null).then((_) {
+      expect(changes, 0);
+      o2.a.b = 4;
+    }).then(_nextMicrotask).then((_) {
+      expect(changes, 1);
+      expect(list[1].a.b, 4);
+      o1.a = new TestModel()..b = 5;
+    }).then(_nextMicrotask).then((_) {
+      expect(changes, 2);
+      expect(list[0].a.b, 5);
+    });
+  });
+}
+
+_nextMicrotask(_) => new Future(() {});
+
+@reflectable
+class TestModel extends ChangeNotifier {
+  var _a, _b;
+  TestModel();
+
+  get a => _a;
+  void set a(newValue) { _a = notifyPropertyChange(#a, _a, newValue); }
+
+  get b => _b;
+  void set b(newValue) { _b = notifyPropertyChange(#b, _b, newValue); }
+}