Mark class as abstract

src/Composer/IO/BaseIO.php

@@ -17,7 +17,7 @@ use Symfony\Component\Console\Output\OutputInterface;
 use Symfony\Component\Console\Helper\HelperSet;
 use Composer\Config;
 
-class BaseIO implements IOInterface
+abstract class BaseIO implements IOInterface
 {
     protected $authentications = array();