8 files declare their use of UserInterface

Entity.php in drupal/core/lib/Drupal/Core/Entity/Entity.php
Contains \Drupal\Core\Entity\Entity.
EntityApiTest.php in drupal/core/modules/system/lib/Drupal/system/Tests/Entity/EntityApiTest.php
Definition of Drupal\system\Tests\Entity\EntityApiTest.
EntityTranslationWorkflowsTest.php in drupal/core/modules/translation_entity/lib/Drupal/translation_entity/Tests/EntityTranslationWorkflowsTest.php
Contains \Drupal\entity\Tests\EntityTranslationWorkflowsTest.
MessageFormController.php in drupal/core/modules/contact/lib/Drupal/contact/MessageFormController.php
Definition of Drupal\contact\MessageFormController.
node.module in drupal/core/modules/node/node.module
The core module that allows content to be submitted to the site.
user.module in drupal/core/modules/user/user.module
Enables the user registration and login system.
User.php in drupal/core/modules/user/lib/Drupal/user/Plugin/Core/Entity/User.php
Definition of Drupal\user\Plugin\Core\Entity\User.
UserBulkForm.php in drupal/core/modules/user/lib/Drupal/user/Plugin/views/field/UserBulkForm.php
Contains \Drupal\user\Plugin\views\field\UserBulkForm.