anchor.diff:  _runTests() {
     var list;
     var obs;
  -  var o1, o2, o3;
  +  var o1, o2;
     var sub;
     int changes;
   
  
positive.code: 
    tearDown(() {
      sub.cancel();
      list = obs = o1 = o2 = o3 = null;
    });
  
    test('list path observer noticed length changes', () {
  
negative.code:     list = toObservable([
        o1 = new TestModel()..a = (new TestModel()..b = 1),
        o2 = new TestModel()..a = (new TestModel()..b = 2),
        new TestModel()..a = (new TestModel()..b = 3)
      ]);
      obs = new ListPathObserver(list, 'a.b');
      changes = 0;
  