Split CI testing based on flutter channels (#651)

experimental/null_safety/null_safe_app/lib/badapp.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+// ignore_for_file: unused_local_variable
+
 import 'package:flutter/material.dart';
 
 import 'services.dart';