ContextualLinkManager::$defaults

protected $defaults = array(
  // (required) The name of the route to link to.
  'route_name' => '',
  // (required) The contextual links group.
  'group' => '',
  // The static title text for the link.
  'title' => '',
  // The default link options.
  'options' => array(),
  // The weight of the link.
  'weight' => NULL,
  // Default class for contextual link implementations.
  'class' => '\Drupal\Core\Menu\ContextualLinkDefault',
  // The plugin id. Set by the plugin system based on the top-level YAML key.
  'id' => '',
);