9 files declare their use of ContentEntityInterface

CommentInterface.php in drupal/core/modules/comment/lib/Drupal/comment/CommentInterface.php
Contains \Drupal\comment\Plugin\Core\Entity\CommentInterface.
CustomBlockInterface.php in drupal/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockInterface.php
Contains \Drupal\custom_block\Plugin\Core\Entity\CustomBlockInterface.
FeedInterface.php in drupal/core/modules/aggregator/lib/Drupal/aggregator/FeedInterface.php
Contains \Drupal\aggregator\Plugin\Core\Entity\FeedInterface.
FileInterface.php in drupal/core/modules/file/lib/Drupal/file/FileInterface.php
Contains \Drupal\file\Plugin\Core\Entity\FileInterface.
ItemInterface.php in drupal/core/modules/aggregator/lib/Drupal/aggregator/ItemInterface.php
Contains \Drupal\aggregator\Plugin\Core\Entity\ItemInterface.
MenuLink.php in drupal/core/modules/menu_link/lib/Drupal/menu_link/Plugin/Core/Entity/MenuLink.php
Contains \Drupal\menu_link\Plugin\Core\Entity\MenuLink.
MenuLinkInterface.php in drupal/core/modules/menu_link/lib/Drupal/menu_link/MenuLinkInterface.php
Contains \Drupal\menu_link\Plugin\Core\Entity\MenuLinkInterface.
NodeInterface.php in drupal/core/modules/node/lib/Drupal/node/NodeInterface.php
Contains \Drupal\node\Plugin\Core\Entity\NodeInterface.
TermInterface.php in drupal/core/modules/taxonomy/lib/Drupal/taxonomy/TermInterface.php
Contains \Drupal\taxonomy\Plugin\Core\Entity\TermInterface.