anchor.diff:      });
     });
   }
  +
  +class _TestClass {}
  
positive.code:         key = null;
        });
  
        test('remove', () async {
          var eventFuture = ref.onValue.first;
  
          await ref.remove();
  
negative.code:         expect(event.snapshot.val()["text"], "ahoj");
        });
  
        test("wrong value push", () {
          expect(() => ref.push(new _TestClass()), throwsArgumentError);
        });
  
        test("transaction", () async {
          var childRef = ref.child("todos");
          childRef.set("Cooking");
  
  