11 files declare their use of StorageInterface

config.admin.inc in drupal/core/modules/config/config.admin.inc
Admin page callbacks for the config module.
config.inc in drupal/core/includes/config.inc
This is the API for configuration storage.
ConfigController.php in drupal/core/modules/config/lib/Drupal/config/Controller/ConfigController.php
Contains \Drupal\config\Controller\ConfigController
ConfigSnapshotSubscriber.php in drupal/core/lib/Drupal/Core/EventSubscriber/ConfigSnapshotSubscriber.php
Contains \Drupal\Core\EventSubscriber\ConfigSnapshotSubscriber.
ConfigStorageController.php in drupal/core/lib/Drupal/Core/Config/Entity/ConfigStorageController.php
Definition of Drupal\Core\Config\Entity\ConfigStorageController.
FieldInstanceStorageController.php in drupal/core/modules/field/lib/Drupal/field/FieldInstanceStorageController.php
Contains \Drupal\field\FieldInstanceStorageController.
FieldStorageController.php in drupal/core/modules/field/lib/Drupal/field/FieldStorageController.php
Contains \Drupal\field\FieldStorageController.
LocaleConfigManager.php in drupal/core/modules/locale/lib/Drupal/locale/LocaleConfigManager.php
Contains \Drupal\locale\LocaleConfigManager.
Query.php in drupal/core/lib/Drupal/Core/Config/Entity/Query/Query.php
Contains \Drupal\Core\Config\Entity\Query\Query.
QueryFactory.php in drupal/core/lib/Drupal/Core/Config/Entity/Query/QueryFactory.php
Contains \Drupal\Core\Config\Entity\Query\QueryFactory.
TestBase.php in drupal/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
Definition of Drupal\simpletest\TestBase.