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

[yaffs-website] / vendor / consolidation / output-formatters / src / Formatters / FormatterAwareInterface.php

<?php
namespace Consolidation\OutputFormatters\Formatters;

interface FormatterAwareInterface
{
    public function setFormatter(FormatterInterface $formatter);
    public function getFormatter();
    public function isHumanReadable();
}