Merge branch '5.1' into 5.x

* 5.1:
  Missing Namespace

Author: OskarStark

event_dispatcher.rst

@@ -284,6 +284,8 @@ This alias mapping can be extended for custom events by registering the
 compiler pass ``AddEventAliasesPass``::
 
     // src/Kernel.php
+    namespace App;
+
     use App\Event\MyCustomEvent;
     use Symfony\Component\DependencyInjection\ContainerBuilder;
     use Symfony\Component\EventDispatcher\DependencyInjection\AddEventAliasesPass;