Mark all dispatched event classes as final

Author: Tobion

CHANGELOG.md

@@ -19,6 +19,7 @@ CHANGELOG
    `Symfony\Component\Mailer\Transport\TransportFactoryInterface` and tagging with `mailer.transport_factory` tag in DI.
  * Added `Symfony\Component\Mailer\Test\TransportFactoryTestCase` to ease testing custom transport factories.
  * Added `SentMessage::getDebug()` and `TransportExceptionInterface::getDebug` to help debugging
+ * Made `MessageEvent` final
 
 4.3.0
 -----

Event/MessageEvent.php

@@ -20,7 +20,7 @@
  *
  * @author Fabien Potencier <fabien@symfony.com>
  */
-class MessageEvent extends Event
+final class MessageEvent extends Event
 {
     private $message;
     private $envelope;