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

[yaffs-website] / vendor / symfony / dependency-injection / Tests / Fixtures / xml / extensions / services5.xml

<?xml version="1.0" ?>

<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:project="http://www.example.com/schema/projectwithxsd"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd
                        http://www.example.com/schema/projectwithxsd http://www.example.com/schema/projectwithxsd/project-1.0.xsd">

    <project:foobar />

</container>