anchor.diff:  
     tearDown(() {
       sub.cancel();
  -    list = obs = o1 = o2 = o3 = null;
  +    list = obs = o1 = o2 = null;
     });
   
     test('list path observer noticed length changes', () {
  
positive.code:     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;
  
negative.code: _runTests() {
    var list;
    var obs;
    var o1, o2;
    var sub;
    int changes;
  
  