make it possible to decorate services

[xabbuh]

If another part of the application decorates a built-in service, the
compiler passes will fail if they run after service decorators have
been resolved (in this case, the original service id is an alias
pointing to the decorating service).

[xabbuh]

In FOSRestbundle, for example, we currently have a weird workaround which replaces the jms_serializer.handler_registry service with a custom definition after the JMSSerializerBundle passes have been processed. However, this will not work anymore in Symfony 4 where services are private by default (because then, at the time when the pass is run, the JMS services may have been inilined because they are private).

[goetas]

👍

[xabbuh]

Thanks for your quick reaction @goetas! This will greatly simplify things on our end. Much appreciated!