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

vendor/symfony/console/Tests/Fixtures/Foo2Command.php

@@ -0,0 +1,21 @@
+<?php
+
+use Symfony\Component\Console\Command\Command;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+class Foo2Command extends Command
+{
+    protected function configure()
+    {
+        $this
+            ->setName('foo1:bar')
+            ->setDescription('The foo1:bar command')
+            ->setAliases(array('afoobar2'))
+        ;
+    }
+
+    protected function execute(InputInterface $input, OutputInterface $output)
+    {
+    }
+}