13 files declare their use of UnknownExtensionException

ExtensionList.php in core/lib/Drupal/Core/Extension/ExtensionList.php
ExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
ExtensionPathResolver.php in core/lib/Drupal/Core/Extension/ExtensionPathResolver.php
ExtensionPathResolverTest.php in core/tests/Drupal/KernelTests/Core/Bootstrap/ExtensionPathResolverTest.php
ModuleHandler.php in core/lib/Drupal/Core/Extension/ModuleHandler.php
ModuleHandlerTest.php in core/tests/Drupal/KernelTests/Core/Extension/ModuleHandlerTest.php
ModuleHandlerTest.php in core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php
ReviewForm.php in core/modules/migrate_drupal_ui/src/Form/ReviewForm.php
system.post_update.php in core/modules/system/system.post_update.php
Post update functions for System.
ThemeHandler.php in core/lib/Drupal/Core/Extension/ThemeHandler.php
ThemeInstaller.php in core/lib/Drupal/Core/Extension/ThemeInstaller.php
ThemeInstallerTest.php in core/tests/Drupal/KernelTests/Core/Theme/ThemeInstallerTest.php
update.inc in core/includes/update.inc
Drupal database update API.

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