8 files declare their use of DrupalKernelInterface in Drupal 9

DrupalKernelTest.php in core/tests/Drupal/KernelTests/Core/DrupalKernel/DrupalKernelTest.php

KernelPreHandle.php in core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php

LanguageRequestSubscriber.php in core/modules/language/src/EventSubscriber/LanguageRequestSubscriber.php

ModifyServiceDefinitionsPass.php in core/lib/Drupal/Core/DependencyInjection/Compiler/ModifyServiceDefinitionsPass.php

ModuleInstaller.php in core/lib/Drupal/Core/Extension/ModuleInstaller.php

ModuleInstallerDeprecationTest.php in core/tests/Drupal/KernelTests/Core/Extension/ModuleInstallerDeprecationTest.php

ServerCommand.php in core/lib/Drupal/Core/Command/ServerCommand.php

UpdateRootFactory.php in core/modules/update/src/UpdateRootFactory.php