anchor.diff:  
       setUp(() {
         list = toObservable([1, 2, 3, 1, 3, 4]);
  -      rol = new ReadOnlyObservableList(list);
  +      rol = factory(list);
         records = null;
         sub = rol.changes.listen((r) { records = r; });
       });
  
positive.code: 
    observeTest('toString', () {
      var list = toObservable([1, 2, 3]);
      var rol = new ReadOnlyObservableList(list);
      expect(rol.toString(), '[1, 2, 3]');
    });
  
  
negative.code: 
      setUp(() {
        list = toObservable([1, 2, 3]);
        rol = factory(list);
        changes = null;
        sub = rol.changes.listen((records) {
          changes = records.where((r) => r.changes(_LENGTH)).toList();
  