12 files declare their use of EntityHandlerInterface in Drupal 8

BlockAccessControlHandler.php in core/modules/block/src/BlockAccessControlHandler.php

BlockContentAccessControlHandler.php in core/modules/block_content/src/BlockContentAccessControlHandler.php

ContentTranslationHandler.php in core/modules/content_translation/src/ContentTranslationHandler.php

DefaultHtmlRouteProvider.php in core/lib/Drupal/Core/Entity/Routing/DefaultHtmlRouteProvider.php

EntityManagerTest.php in core/tests/Drupal/Tests/Core/Entity/EntityManagerTest.php

EntityModerationRouteProvider.php in core/modules/content_moderation/src/Entity/Routing/EntityModerationRouteProvider.php

EntityViewsData.php in core/modules/views/src/EntityViewsData.php

MenuLinkContentAccessControlHandler.php in core/modules/menu_link_content/src/MenuLinkContentAccessControlHandler.php

ModerationHandler.php in core/modules/content_moderation/src/Entity/Handler/ModerationHandler.php

NodeAccessControlHandler.php in core/modules/node/src/NodeAccessControlHandler.php

ShortcutAccessControlHandler.php in core/modules/shortcut/src/ShortcutAccessControlHandler.php

WorkflowAccessControlHandler.php in core/modules/workflows/src/WorkflowAccessControlHandler.php