20 files declare their use of ExtensionDiscovery

DrupalKernel.php in core/lib/Drupal/Core/DrupalKernel.php
EntityFilteringThemeTest.php in core/modules/system/tests/src/Functional/Theme/EntityFilteringThemeTest.php
ExtensionDiscoveryTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
ExtensionInstallStorage.php in core/lib/Drupal/Core/Config/ExtensionInstallStorage.php
ExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php
ExtensionNameConstraint.php in core/lib/Drupal/Core/Extension/Plugin/Validation/Constraint/ExtensionNameConstraint.php
FieldTypePluginManagerTest.php in core/modules/field/tests/src/Kernel/FieldTypePluginManagerTest.php
GenerateTheme.php in core/lib/Drupal/Core/Command/GenerateTheme.php
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.
InstallCommand.php in core/lib/Drupal/Core/Command/InstallCommand.php
InstallStorage.php in core/lib/Drupal/Core/Config/InstallStorage.php
KernelTestBase.php in core/tests/Drupal/KernelTests/KernelTestBase.php
SystemListingTest.php in core/modules/system/tests/src/Kernel/Common/SystemListingTest.php
TestDiscovery.php in core/lib/Drupal/Core/Test/TestDiscovery.php
TestInstallStorage.php in core/modules/config/tests/config_test/src/TestInstallStorage.php
TestRequirementsTrait.php in core/tests/Drupal/Tests/TestRequirementsTrait.php
ThemeExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
ThemeSettingsTest.php in core/tests/Drupal/KernelTests/Core/Theme/ThemeSettingsTest.php
UpdateRegistry.php in core/lib/Drupal/Core/Update/UpdateRegistry.php

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