title: Flutter - change appbar icon when receiving notification
body: <p>I am using FirebaseMessaging to push notifications on my app. </p>
  
  <p>So I can handle these notification with this code :</p>
  
  <pre><code>firebaseMessaging.configure(
      onLaunch: (Map&lt;String, dynamic&gt; msg) {
    print("onLaunch called");
  }, onResume: (Map&lt;String, dynamic&gt; msg) {
    print("onResume called");
  }, onMessage: (Map&lt;String, dynamic&gt; msg) {
    print("onMessage called : " + msg.toString());
  });
  </code></pre>
  
  <p>When I receive a notification, I want to display this little '1' on my icon in my appbar</p>
  
  <p><a href="https://i.stack.imgur.com/NWf5k.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/NWf5k.jpg" alt="icon"></a></p>
  
  <p>My problem is : I don't know how to change my bell icon dynamically on my appbar for all pages (and I can't call setState in my appbar)</p>