29 files declare their use of Extension

ComponentNegotiator.php in core/lib/Drupal/Core/Theme/ComponentNegotiator.php
DrupalKernel.php in core/lib/Drupal/Core/DrupalKernel.php
ExtensionDiscoveryTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
ExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
ExtensionSerializationTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionSerializationTest.php
FieldDefinitionIntegrityTest.php in core/modules/field/tests/src/Kernel/FieldDefinitionIntegrityTest.php
GenerateTheme.php in core/lib/Drupal/Core/Command/GenerateTheme.php
InstallerNonDefaultDatabaseDriverTest.php in core/tests/Drupal/FunctionalTests/Installer/InstallerNonDefaultDatabaseDriverTest.php
InstallStorage.php in core/lib/Drupal/Core/Config/InstallStorage.php
LayoutPluginManagerTest.php in core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php
LinkRelationTypeManager.php in core/lib/Drupal/Core/Http/LinkRelationTypeManager.php
ModuleHandlerTest.php in core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php
ModulesListForm.php in core/modules/system/src/Form/ModulesListForm.php
module_test.module in core/modules/system/tests/modules/module_test/module_test.module
Test module.
PermissionHandlerTest.php in core/modules/user/tests/src/Unit/PermissionHandlerTest.php
ProjectInfo.php in core/lib/Drupal/Core/Utility/ProjectInfo.php
ResourceResponseValidatorTest.php in core/modules/jsonapi/tests/src/Unit/EventSubscriber/ResourceResponseValidatorTest.php
SecurityAdvisoriesFetcherTest.php in core/modules/system/tests/src/Kernel/SecurityAdvisories/SecurityAdvisoriesFetcherTest.php
system.module in core/modules/system/system.module
Configuration system that lets administrators modify the workings of the site.
SystemLocalTasksTest.php in core/modules/system/tests/src/Unit/Menu/SystemLocalTasksTest.php
system_test.module in core/modules/system/tests/modules/system_test/system_test.module
Test module.
TestDiscoveryTest.php in core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php
TestRunnerKernel.php in core/lib/Drupal/Core/Test/TestRunnerKernel.php
ThemeExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
ThemeHandlerTest.php in core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
ThemeInitialization.php in core/lib/Drupal/Core/Theme/ThemeInitialization.php
ThemeInitializationInterface.php in core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php
theme_test.module in core/modules/system/tests/modules/theme_test/theme_test.module
Test module.
UpdateRegistry.php in core/lib/Drupal/Core/Update/UpdateRegistry.php

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