18 files declare their use of ConfigImporter in Drupal 9

ConfigImporterBatch.php in core/lib/Drupal/Core/Config/Importer/ConfigImporterBatch.php

ConfigImporterFieldPurger.php in core/modules/field/src/ConfigImporterFieldPurger.php

ConfigImporterMissingContentTest.php in core/tests/Drupal/KernelTests/Core/Config/ConfigImporterMissingContentTest.php

ConfigImporterTest.php in core/tests/Drupal/KernelTests/Core/Config/ConfigImporterTest.php

ConfigImportRecreateTest.php in core/tests/Drupal/KernelTests/Core/Config/ConfigImportRecreateTest.php

ConfigImportRenameValidationTest.php in core/tests/Drupal/KernelTests/Core/Config/ConfigImportRenameValidationTest.php

ConfigImportSubscriber.php in core/lib/Drupal/Core/EventSubscriber/ConfigImportSubscriber.php

ConfigSingleImportForm.php in core/modules/config/src/Form/ConfigSingleImportForm.php

ConfigSync.php in core/modules/config/src/Form/ConfigSync.php

ConfigTestTrait.php in core/tests/Drupal/Tests/ConfigTestTrait.php

ConfigUninstallViaCliImportTest.php in core/modules/config/tests/src/Kernel/ConfigUninstallViaCliImportTest.php

config_import_test.module in core/modules/config/tests/config_import_test/config_import_test.module

Provides configuration import test helpers.

ContentEntityNullStorageTest.php in core/tests/Drupal/KernelTests/Core/Entity/ContentEntityNullStorageTest.php

ContentTranslationConfigImportTest.php in core/modules/content_translation/tests/src/Kernel/ContentTranslationConfigImportTest.php

field.module in core/modules/field/field.module

Attach custom data fields to Drupal entities.

ImportStorageTransformerTest.php in core/tests/Drupal/KernelTests/Core/Config/ImportStorageTransformerTest.php

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

API functions for installing Drupal.

OverriddenConfigImportTest.php in core/modules/language/tests/src/Kernel/OverriddenConfigImportTest.php