title: @angular/fire AngularFireStorage getDownloadUrl()
body: <p>I am trying to use the new @angular/fire 5.1.0
  to view an image uploaded to firebase using AngularFireStorage.
  I used to be able to make use of <code>task.downloadURL()</code> in angularfire2</p>
  
  <p>"correct me if I'm wrong but now I have to use <code>afStorage.getDownloadURL()</code>
  Please can you kindly assist me with setting imageUrl correctly.</p>
  
  <pre><code>import { AngularFireStorage, AngularFireStorageReference, AngularFireUploadTask } from '@angular/fire/storage'
  ....
          downloadURL: Observable&lt;string&gt;; 
          imageUrl: string;
  ....
  
  async onGetFromGallery(){
    try{ ....
      const imageData = await this.camera.getPicture(options);
      const image = 'data:image/jpeg;base64,' + imageData;
      const id = Math.random().toString(36).substring(2);
      const user = this.authenticatorService.getUser();
      this.ref = this.afStorage.ref(user.uid + '/images/categories/'+id);
      this.task = this.ref.putString(image, 'data_url');
  
      //--- Previously:angularfire2:
      // this.downloadURL = this.ref.getDownloadURL();
      // this.downloadURL.subscribe(url=&gt; this.imageUrl=url);
      //--- now replaced by this.ref.getDownloadURL()...
  
      //My Attempt - unable to getDownloadUrl?
      this.task
      .snapshotChanges().toPromise().then(_ =&gt;
         {
          this.ref.getDownloadURL().toPromise().then(res =&gt; {
            console.log('URL: ', res);
            this.imageUrl = res;
          });
        })
  
    } catch(e) {
      console.error(e);
      this.errorMessage = JSON.stringify(e);
    }
  }
  </code></pre>
  
  <p>view excerpt:</p>
  
  <pre><code>&lt;img [src]="imageUrl"  style="width:100%;"&gt;
  </code></pre>
  
  <p>package.json excerpt:</p>
  
  <blockquote>
    <p>"@angular/compiler-cli": "5.2.11", <br />
    "@angular/fire": "^5.1.0", <br />
    "firebase": "^5.5.9", <br />
    "cordova-android": "~7.0.0", <br /></p>
  </blockquote>
  
  <p>Thank you.</p>