anchor.diff:      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)
  +      new TestModel()..a = (new TestModel()..b = 3)
       ]);
       obs = new ListPathObserver(list, 'a.b');
       changes = 0;
  
positive.code: _runTests() {
    var list;
    var obs;
    var o1, o2, o3;
    var sub;
    int changes;
  
  
negative.code: 
    tearDown(() {
      sub.cancel();
      list = obs = o1 = o2 = null;
    });
  
    test('list path observer noticed length changes', () {
  