title: AngularFire Storage get file url
body: <p>I have a problem with getDownloadURL() method from AngularFire Storage. I couldn't use then() without an error.</p>
  
  <p>Here is my code :</p>
  
  <pre><code>import { Component } from '@angular/core';
  import { NavController } from 'ionic-angular';
  
  import { AngularFireModule } from 'angularfire2';
  import { AngularFireDatabase } from 'angularfire2/database';
  import { AngularFireStorage } from 'angularfire2/storage';
  
  import { GroupDetailsPage } from '../group-details/group-details';
  
  @Component({
    selector: 'page-home',
    templateUrl: 'home.html'
  })
  export class HomePage {
  
      constructor(public navCtrl: NavController, private afDB: AngularFireDatabase, private afStorage: AngularFireStorage) {
  
  
          let ref = this.afStorage.ref('test.jpg');
              ref.getDownloadURL().then(function(url){
                  console.log( url );
              });
  
      }
  
  }
  </code></pre>
  
  <p>Here my error :
  ERROR Error: Uncaught (in promise): TypeError: ref.getDownloadURL(...).then is not a function</p>
  
  <p>If I don't use then() I get a PromiseObservable as response.</p>
  
  <p>Have you got an idea ?</p>
  
  <p>Thanks in advance,</p>