constructor(public af:AngularFire) {
   var ref = new Firebase(rootFirebase);
   ref.remove();
   this.questions = af.list('/questions');
   this.questions.add({question: 'why?'});
   this.questions.add({question: 'how?'});
 }

 constructor(public af:AngularFire) {
   var ref = new Firebase(rootFirebase);
   ref.remove();
   this.question = af.object('/questions/1');
   ref.child('/questions/1').set({title: 'how to firebase?'});
 }

 afterEach((done:any) => {
   ref.off();
   ref.remove();
   done();
 });

 delete(event) {
   let url = this._endpoint + event.$key;
   let item = new Firebase(this._endpoint + event.$key);
   item.remove();
 }