title: function doesn't work within the firebase Auth with Angular
body: <p>I'm trying to use Firebase Auth with my Angular7 app.
  I want to use some function to process user data.
  However after I login with google account, codes until myFunction works,
  but after myFunction doesn't work
  I'm not sure the root cause as no message was shown.</p>
  
  <p>Could you please advice to me?</p>
  
  <pre><code>constructor(private afAuth: AngularFireAuth) {}
  
  public myFunction(obj: any){
    console.log(obj)
  }
  
  public socialLogin() {
    var provider = new firebase.auth.GoogleAuthProvider();
    this.afAuth.auth.useDeviceLanguage();
    this.afAuth.auth.signInWithPopup(provider).then(function(result) {
      console.log('logged in'); &lt;- work!
      var user = result.user; &lt;- work!
      console.log(user) &lt;- work!
      this.myFunction(user); &lt;- not work
      console.log('test'); &lt;- not work
    })
  </code></pre>
  
  <p>typescript version is 2.9.2</p>