Fix errors found by PHPStan

Test Plan:
Ran phpstan analyze --autoload-file=src/__phutil_library_init__.php -c phpstan.neon -l 1 src/ with phpstan.neon containing:

parameters:
    excludes_analyse:
        - */__tests__/*

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17364