Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / vendor / drush / drush / src / Drupal / DrupalKernel.php

<?php

namespace Drush\Drupal;

use Drupal\Core\DrupalKernel as DrupalDrupalKernel;

/**
 * Overridden version of DrupalKernel adapted to the needs of Drush.
 */
class DrupalKernel extends DrupalDrupalKernel
{
    use DrupalKernelTrait;
}