Extension compiler pass cannot find class

If an extension tries to add a custom compiler pass via /di/pass/*_pass.php, the class can't be found.

Steps to reproduce:
1. Create an extension, give it the compiler pass file: eg. /foo/bar/di/pass/test_pass.php
2. Load the extension.

Broken line:
$filename = './ext//foo/bar/di/pass/test_pass'
class_exists('\\foo\bar\di\pass\test_pass')