Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/symfony/event-dispatcher/README.md

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+EventDispatcher Component
+=========================
+
+The Symfony EventDispatcher component implements the Mediator pattern in a
+simple and effective way to make your projects truly extensible.
+
+```php
+use Symfony\Component\EventDispatcher\EventDispatcher;
+use Symfony\Component\EventDispatcher\Event;
+
+$dispatcher = new EventDispatcher();
+
+$dispatcher->addListener('event_name', function (Event $event) {
+    // ...
+});
+
+$dispatcher->dispatch('event_name');
+```
+
+Resources
+---------
+
+You can run the unit tests with the following command:
+
+    $ cd path/to/Symfony/Component/EventDispatcher/
+    $ composer install
+    $ phpunit