Fix analyze errors

Author: kroikie

packages/firebase_messaging/README.md

@@ -96,12 +96,12 @@ By default background messaging is not enabled. To handle messages in the backgr
     Future<dynamic> myBackgroundMessageHandler(Map<String, dynamic> message) {
       if (message.containsKey('data')) {
         // Handle data message
-        dynamic data = message['data'];
+        final dynamic data = message['data'];
       }
     
       if (message.containsKey('notification')) {
         // Handle notification message
-        dynamic notification = message['notification'];
+        final dynamic notification = message['notification'];
       }
     
       // Or do other work.

packages/firebase_messaging/test/firebase_messaging_test.dart

@@ -15,12 +15,10 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   MockMethodChannel mockChannel;
-  MockMethodChannel mockBackgroundChannel;
   FirebaseMessaging firebaseMessaging;
 
   setUp(() {
     mockChannel = MockMethodChannel();
-    mockBackgroundChannel = MockMethodChannel();
     firebaseMessaging = FirebaseMessaging.private(
         mockChannel, FakePlatform(operatingSystem: 'ios'));
   });