When we updated to Symfony 2.8 in #2611816: Update to symfony 2.8 we didn't implement the getListenerPriority() method on ContainerAwareEventDispatcher, probably because it was not required on the interface.
Unfortunately the omission of this method on the interface is a bug in Symfony, since the method is being actively used. This has been fixed in Symfony 3.0 in Issue #16301: [EventDispatcher] add method getListenerPriority() to interface but it is not clear if this will be backported to 2.8.
Marking this as a bug since it is already breaking contrib (e.g. #2682343: Fatal error enabling web profiler on 8.1.x). We'll have to implement this anyway before Drupal 8.2.x which will depend on Symfony 3.0, but it would be good to have this in earlier.
