Symfony version(s) affected: 4.2@dev
Work with 3.4, 4.0 and 4.1 without message.

Description
1x: The "Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait::loadRoutes()" method is considered internal. It may change without further notice. You should not extend it from "App\Kernel".
1x in ExtensionTest::testService from Mazarini\ToolsBundle\Tests\Controller\DependencyInjection

How to reproduce
Probably append when "$this->bootKernel();" during test
I don't extend it, it's seems append with unmodified kernel.