MenuLinkManager::$defaults

protected $defaults = array(
  // (required) The name of the menu for this link.
  'menu_name' => 'tools',
  // (required) The name of the route this links to, unless it's external.
  'route_name' => '',
  // Parameters for route variables when generating a link.
  'route_parameters' => array(),
  // The external URL if this link has one (required if route_name is empty).
  'url' => '',
  // The static title for the menu link. If this came from a YAML definition
  // or other safe source this may be a TranslatableMarkup object.
  'title' => '',
  // The description. If this came from a YAML definition or other safe source
  // this may be be a TranslatableMarkup object.
  'description' => '',
  // The plugin ID of the parent link (or NULL for a top-level link).
  'parent' => '',
  // The weight of the link.
  'weight' => 0,
  // The default link options.
  'options' => array(),
  'expanded' => 0,
  'enabled' => 1,
  // The name of the module providing this link.
  'provider' => '',
  'metadata' => array(),
  // Default class for local task implementations.
  'class' => 'Drupal\Core\Menu\MenuLinkDefault',
  'form_class' => 'Drupal\Core\Menu\Form\MenuLinkDefaultForm',
  // The plugin ID. Set by the plugin system based on the top-level YAML key.
  'id' => '',
);