title: Firebase 3 - ref.getAuth() equivalent
body: <p>In Firebase 2 I was able to do a synchronous check to see if the user was logged in, e.g:</p>
  
  <pre><code>if (!ref.getAuth()) ..go to login page
  else ..show page
  </code></pre>
  
  <p>I can access the <code>currentUser</code> state with <code>firebase.auth().currentUser</code> so I tried to do:</p>
  
  <pre><code>if (!firebase.auth().currentUser) ..go to login page
  else ..show page
  </code></pre>
  
  <p>The problem is that this is always initially null, so it redirects to the login page, then a few moments later <code>onAuthStateChanged</code> happens and the user is logged in.</p>
  
  <p>Is there still a way to synchronously check this? Or otherwise is there a promise I can listen to so that I can show a loading spinner or something? Thanks.</p>