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

[yaffs-website] / web / core / modules / filter / tests / filter_test / filter_test.routing.yml

filter_text.text_format:
  path: '/filter-test/text-format'
  defaults:
    _title: 'Text format test forms'
    _form: 'Drupal\filter_test\Form\FilterTestFormatForm'
  requirements:
    _access: 'TRUE'