title: flutter firebase auth current user email for appbar title
body: <p>User already login to the app by using firebase email login.
  How to make the appbar title to be the current user email?</p>
  
  <pre><code>class _HomePageState extends State&lt;HomePage&gt; {
  
    final  Future&lt;String&gt; userEmail = FirebaseAuth.instance.currentUser().then((FirebaseUser user) =&gt; user.email);
    var e = "";
  
    @override
    Widget build(BuildContext context) {
  
      return new Scaffold(
          appBar: new AppBar(
          backgroundColor: Colors.black,
            title: Text('userEmail'),
            actions: &lt;Widget&gt;[
              new FlatButton(
                  onPressed: _signOut,
                  child: new Text('logout', style: new TextStyle(fontSize: 17.0, color: Colors.white))
              ),
  
            ],
          ),
          body: new Center(
            child: new Text(
              e,
              style: new TextStyle(fontSize: 32.0),
            ),
          )
      );
    }
  }
  </code></pre>