8 files declare their use of ContentNegotiation

AjaxEnhancer.php in drupal/core/lib/Drupal/Core/Routing/Enhancer/AjaxEnhancer.php
Contains \Drupal\Core\Routing\Enhancer\AjaxEnhancer.
ContentControllerEnhancer.php in drupal/core/lib/Drupal/Core/Routing/Enhancer/ContentControllerEnhancer.php
Contains \Drupal\Core\Routing\Enhancer\ContentControllerEnhancer.
EntityFormEnhancer.php in drupal/core/lib/Drupal/Core/Entity/Enhancer/EntityFormEnhancer.php
Contains \Drupal\Core\Entity\Enhancer\EntityFormEnhancer.
ExceptionController.php in drupal/core/lib/Drupal/Core/Controller/ExceptionController.php
Contains \Drupal\Core\Controller\ExceptionController.
ExceptionControllerTest.php in drupal/core/modules/system/lib/Drupal/system/Tests/System/ExceptionControllerTest.php
FormEnhancer.php in drupal/core/lib/Drupal/Core/Routing/Enhancer/FormEnhancer.php
Contains \Drupal\Core\Routing\Enhancer\FormEnhancer.
OverlaySubscriber.php in drupal/core/modules/overlay/lib/Drupal/overlay/EventSubscriber/OverlaySubscriber.php
Contains \Drupal\overlay\EventSubscriber\OverlaySubscriber.
ViewSubscriber.php in drupal/core/lib/Drupal/Core/EventSubscriber/ViewSubscriber.php
Definition of Drupal\Core\EventSubscriber\ViewSubscriber.