title: The getter 'AuthStateChanges' isn't defined for the type 'FirebaseAuth'
body: <p>I'm trying to get UserProvider for Firebase Auth but the error says:</p>
  <blockquote>
  <p>The getter 'AuthStateChanges' isn't defined for the type 'FirebaseAuth'.</p>
  </blockquote>
  <p>and</p>
  <blockquote>
  <p>Undefined name '_onStateChanged'.</p>
  </blockquote>
  <pre><code>UserProvider.initialize() : _auth = FirebaseAuth.instance {
  _auth.AuthStateChanges.listen(_onStateChanged);}
  </code></pre>