When PhpUnit is imported from the phar release, the class SebastianBergmann\Exporter\Exporter is vendored as PhpUnit\SebastianBergmann\Exporter\Exporter.

The DefinitionHasArgumentConstraint in this bundle has a property of type SebastianBergmann\Exporter\Exporter which cause tests to fail, because the autoloader can't find it.