fixed deprecation warnings on calling setMockMethodCallHandler() (#2039)

Author: MaikuB

flutter_local_notifications/test/android_flutter_local_notifications_test.dart

@@ -25,8 +25,8 @@ void main() {
     setUp(() {
       debugDefaultTargetPlatformOverride = TargetPlatform.android;
       flutterLocalNotificationsPlugin = FlutterLocalNotificationsPlugin();
-      // ignore: always_specify_types
-      channel.setMockMethodCallHandler((methodCall) async {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .setMockMethodCallHandler(channel, (MethodCall methodCall) async {
         log.add(methodCall);
         if (methodCall.method == 'initialize') {
           return true;
@@ -37,6 +37,7 @@ void main() {
         } else if (methodCall.method == 'getNotificationAppLaunchDetails') {
           return null;
         }
+        return null;
       });
     });
 

flutter_local_notifications/test/ios_flutter_local_notifications_test.dart

@@ -20,8 +20,8 @@ void main() {
     setUp(() {
       debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
       flutterLocalNotificationsPlugin = FlutterLocalNotificationsPlugin();
-      // ignore: always_specify_types
-      channel.setMockMethodCallHandler((methodCall) async {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .setMockMethodCallHandler(channel, (MethodCall methodCall) async {
         log.add(methodCall);
         if (methodCall.method == 'pendingNotificationRequests') {
           return <Map<String, Object>?>[];
@@ -30,6 +30,7 @@ void main() {
         } else if (methodCall.method == 'getNotificationAppLaunchDetails') {
           return null;
         }
+        return null;
       });
     });
 

flutter_local_notifications/test/macos_flutter_local_notifications_test.dart

@@ -19,8 +19,8 @@ void main() {
     setUp(() {
       debugDefaultTargetPlatformOverride = TargetPlatform.macOS;
       flutterLocalNotificationsPlugin = FlutterLocalNotificationsPlugin();
-      // ignore: always_specify_types
-      channel.setMockMethodCallHandler((methodCall) async {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .setMockMethodCallHandler(channel, (MethodCall methodCall) async {
         log.add(methodCall);
         if (methodCall.method == 'pendingNotificationRequests') {
           return <Map<String, Object>?>[];
@@ -29,6 +29,7 @@ void main() {
         } else if (methodCall.method == 'getNotificationAppLaunchDetails') {
           return null;
         }
+        return null;
       });
     });