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

[yaffs-website] / vendor / symfony / dependency-injection / Tests / Fixtures / yaml / services_instanceof_with_parent.yml

services:
    _instanceof:
        FooInterface:
            autowire: true

    parent_service:
        class: stdClass

    child_service:
        class: stdClass
        parent: parent_service