anchor.diff:          childRef.set(["Programming", "Cooking", "Walking with dog"]);
   
           var subscription = childRef.onValue.listen(expectAsync((event) {
  -          var snapshot = event.snapshot;
             var todos = event.snapshot.val();
             expect(todos, isNotNull);
             expect(todos.length, 3);
  
positive.code:         childRef.push("Walking with dog");
  
          var subscription = childRef.onChildRemoved.listen(expectAsync((event) {
            var snapshot = event.snapshot;
            expect(snapshot.val(), "Programming");
            expect(snapshot.val(), isNot("Cooking"));
          }, count: 1));
  
          childRef.child(childKey).remove();
  
negative.code:         var todos = [];
          var eventsCount = 0;
          var subscription = childRef.onChildAdded.listen(expectAsync((event) {
            var val = event.snapshot.val();
            todos.add(val);
            eventsCount++;
            expect(eventsCount, isNonZero);
            expect(eventsCount, lessThan(4));
            expect(val,
                anyOf("Programming", "Cooking", "Walking with dog"));
          }, count: 3));
  
  