title: gapi.auth2.getAuthInstance().isSignedIn.listen() doesn't work
body: <p>I am using OAuth2.0 in google drive api authentication. I have an <code>isSignedIn</code> listener that has <code>afterSignIn</code> as a callback.<br>
  <strong>My problem:</strong> after signing in, <code>afterSignIn()</code> function is not fired. Does someone know how to fix this?</p>
  
  <pre><code>function googleDriveAuthentication($rootScope){
      var API_KEY = '...'
      var CLIENT_ID = '...';
      var SCOPES = 'https://www.googleapis.com/auth/drive';
      this.authenticate = function(){
          gapi.load('client:auth2',authorize);
      }
      function authorize(){
          gapi.client.setApiKey(API_KEY);
          gapi.auth2.init({
              client_id: CLIENT_ID,
              scope: SCOPES
          }).then(function(authResult){
              var auth2 = gapi.auth2.getAuthInstance();
              auth2.isSignedIn.listen(afterSignIn);
              auth2.signIn();
          });
      }
      function afterSignIn(){
          console.log('authenticated successfully');
          $rootScope.authenticated = true;
          $rootScope.$broadcast('authenticated');
          gapi.client.load('drive', 'v3');
      }
  }
  </code></pre>