fix(firebase_messaging): Update notification key to NSApplicationLaunchUserNotificationKey for macOS. (#8251)

Author: russellwheatley

packages/firebase_messaging/firebase_messaging/ios/Classes/FLTFirebaseMessagingPlugin.m

@@ -194,8 +194,13 @@ - (void)messaging:(nonnull FIRMessaging *)messaging
 
 - (void)application_onDidFinishLaunchingNotification:(nonnull NSNotification *)notification {
   // Setup UIApplicationDelegate.
+#if TARGET_OS_OSX
+  NSDictionary *remoteNotification =
+      notification.userInfo[NSApplicationLaunchUserNotificationKey];
+#else
   NSDictionary *remoteNotification =
       notification.userInfo[UIApplicationLaunchOptionsRemoteNotificationKey];
+#endif
   if (remoteNotification != nil) {
     // If remoteNotification exists, it is the notification that opened the app.
     _initialNotification =