title: Property 'currentUser' does not exist on type 'FirebaseAuthState'
body: <p>trying to get currentUser in angularfire2. what type exposes this?</p>
  
  <p><a href="https://firebase.google.com/docs/reference/js/firebase.auth.Auth" rel="nofollow">https://firebase.google.com/docs/reference/js/firebase.auth.Auth</a></p>
  
  <p>it's not obvious to me from exploring this observable:</p>
  
  <pre><code>constructor(private auth: FirebaseAuth) {
    auth.subscribe((user) =&gt; console.log('user is', user));    
  }
  </code></pre>