Fix errors found by PHPStan

Test Plan:
Ran phpstan analyze -a autoload.php arcanist/src with autoload.php containing:

<?php
require_once 'libphutil/src/__phutil_library_init__.php';
require_once 'arcanist/src/__phutil_library_init__.php';

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17367