15 files declare their use of ExtensionDiscovery in Zircon Profile 8

DrupalKernel.php in core/lib/Drupal/Core/DrupalKernel.php

Contains \Drupal\Core\DrupalKernel.

EntityFilteringThemeTest.php in core/modules/system/src/Tests/Theme/EntityFilteringThemeTest.php

Contains \Drupal\system\Tests\Theme\EntityFilteringThemeTest.

ExtensionInstallStorage.php in core/lib/Drupal/Core/Config/ExtensionInstallStorage.php

Contains \Drupal\Core\Config\ExtensionInstallStorage.

install.core.inc in core/includes/install.core.inc

API functions for installing Drupal.

install.inc in core/includes/install.inc

API functions for installing modules and themes.

InstallStorage.php in core/lib/Drupal/Core/Config/InstallStorage.php

Contains \Drupal\Core\Config\InstallStorage.

KernelTestBase.php in core/tests/Drupal/KernelTests/KernelTestBase.php

Contains \Drupal\KernelTests\KernelTestBase.

KernelTestBase.php in core/modules/simpletest/src/KernelTestBase.php

Contains \Drupal\simpletest\KernelTestBase.

module.inc in core/includes/module.inc

API for loading and interacting with Drupal modules.

system.module in core/modules/system/system.module

Configuration system that lets administrators modify the workings of the site.

SystemListingTest.php in core/modules/system/src/Tests/Common/SystemListingTest.php

Contains \Drupal\system\Tests\Common\SystemListingTest.

TestDiscovery.php in core/modules/simpletest/src/TestDiscovery.php

Contains \Drupal\simpletest\TestDiscovery.

TestInstallStorage.php in core/modules/config/tests/config_test/src/TestInstallStorage.php

Contains \Drupal\config_test\TestInstallStorage.

ThemeSettingsTest.php in core/modules/system/src/Tests/Theme/ThemeSettingsTest.php

Contains \Drupal\system\Tests\Theme\ThemeSettingsTest.

UpdateRegistry.php in core/lib/Drupal/Core/Update/UpdateRegistry.php

Contains \Drupal\Core\Update\UpdateRegistry.