8 files declare their use of UnknownExtensionException in Drupal 8

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

ExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php

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

ModuleHandlerTest.php in core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php

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.