6 files declare their use of HttpKernel

ClientTest.php in drupal/core/vendor/symfony/http-kernel/Symfony/Component/HttpKernel/Tests/ClientTest.php
HttpKernel.php in drupal/core/lib/Drupal/Core/HttpKernel.php
Definition of Drupal\Core\HttpKernel.
HttpKernelTest.php in drupal/core/vendor/symfony/http-kernel/Symfony/Component/HttpKernel/Tests/HttpKernelTest.php
TestHttpKernel.php in drupal/core/vendor/symfony/http-kernel/Symfony/Component/HttpKernel/Tests/TestHttpKernel.php
TestHttpKernel.php in drupal/core/vendor/symfony/http-kernel/Symfony/Component/HttpKernel/Tests/HttpCache/TestHttpKernel.php
TestMultipleHttpKernel.php in drupal/core/vendor/symfony/http-kernel/Symfony/Component/HttpKernel/Tests/HttpCache/TestMultipleHttpKernel.php