8 files declare their use of ImmutableConfig in Drupal 8

ConfigEntityStorageTest.php in core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityStorageTest.php

DateTimeIso8601NormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php

DateTimeNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.php

ImmutableConfigTest.php in core/tests/Drupal/Tests/Core/Config/ImmutableConfigTest.php

PathProcessorFrontTest.php in core/tests/Drupal/Tests/Core/PathProcessor/PathProcessorFrontTest.php

RequestHandlerTest.php in core/modules/rest/tests/src/Kernel/RequestHandlerTest.php

UserRegistrationResource.php in core/modules/user/src/Plugin/rest/resource/UserRegistrationResource.php

UserRegistrationResourceTest.php in core/modules/user/tests/src/Unit/UserRegistrationResourceTest.php