If the menu_link module is not enabled, the following code is run:
<?php
// The Menu link module is not available at install time, so we need to
// hardcode the default storage controller.
$menu_link_controller = new MenuLinkStorageController('menu_link', Drupal::service('database'), Drupal::service('router.route_provider'));
?>but the signature of the constructor has the entity_info as second parameter.
You can't get this information from Drupal::entityManager()->getDefinition('menu_link'), so you might have to copy it from MenuLink.php in the menu_link module.