title: Call onMessage method when the app is in background in flutter
body: <p>I'm new in flutter and dart. I'm trying to connect my app with <a href="https://github.com/flutter/plugins/tree/master/packages/firebase_messaging" rel="noreferrer" title="firebase cloud messaging">FCM</a>. When app is in foreground I create flutterLocalNotificationsPlugin and everything works fine, but I don't how to handle onMessage method when my app is in background. Have somebody any idea how I can resolve it?</p>
  
  <pre><code>FirebaseMessaging firebaseMessaging = new FirebaseMessaging();
  FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin = new FlutterLocalNotificationsPlugin();
  
  @override
  void initState() {
   super.initState();
  
   var androidInitSettings = new AndroidInitializationSettings('mipmap/ic_launcher');
   var iosInitSettings = new IOSInitializationSettings();
   var initSettings = new InitializationSettings(androidInitSettings, iosInitSettings);
   flutterLocalNotificationsPlugin.initialize(initSettings, selectNotification: onSelectNotification);
  
   firebaseMessaging.configure(
     onLaunch: (Map&lt;String, dynamic&gt; msg) {
       print(" onLaunch called ${(msg)}");
     },
     onResume: (Map&lt;String, dynamic&gt; msg) {
       print(" onResume called ${(msg)}");
     },
     onMessage: (Map&lt;String, dynamic&gt; msg) {
       showNotification(msg);
       print(" onMessage called ${(msg)}");
     },
   );
   firebaseMessaging.requestNotificationPermissions(const IosNotificationSettings(sound: true, alert: true, badge: true));
   firebaseMessaging.onIosSettingsRegistered.listen((IosNotificationSettings setting) {
     print('IOS Setting Registed');
   });
   firebaseMessaging.getToken().then((token) {
     update(token);
   });
  }
  </code></pre>