Add a new null safety example (#562)

experimental/null_safety/null_safe_app/lib/main.dart

@@ -0,0 +1,11 @@
+// Copyright 2020, the Flutter project authors. Please see the AUTHORS file
+// 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.
+
+import 'package:flutter/material.dart';
+
+import 'goodapp.dart';
+
+void main() {
+  runApp(GoodMyApp());
+}