7 files declare their use of MenuLinkInterface in Drupal 8

MenuLinkDefaultForm.php in core/lib/Drupal/Core/Menu/Form/MenuLinkDefaultForm.php

MenuLinkEditForm.php in core/modules/menu_ui/src/Form/MenuLinkEditForm.php

MenuLinkFormInterface.php in core/lib/Drupal/Core/Menu/Form/MenuLinkFormInterface.php

MenuLinkResetForm.php in core/modules/menu_ui/src/Form/MenuLinkResetForm.php

MenuUiTest.php in core/modules/menu_ui/tests/src/Functional/MenuUiTest.php

menu_ui.module in core/modules/menu_ui/menu_ui.module

Allows administrators to customize the site's navigation menus.

SystemManager.php in core/modules/system/src/SystemManager.php