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

[yaffs-website] / web / core / modules / contextual / tests / modules / contextual_test / contextual_test.routing.yml

contextual_test:
  path: '/contextual-tests'
  defaults:
    _controller: '\Drupal\contextual_test\Controller\TestController::render'
  requirements:
    _access: 'TRUE'