public static function overrideConfig($key, $value)
 {
     if (!self::$overrideSource) {
         self::$overrideSource = id(new PhabricatorConfigDictionarySource(array()))->setName(pht('Overridden Config'));
         self::$sourceStack->pushSource(self::$overrideSource);
     }
     self::$overrideSource->setKeys(array($key => $value));
     self::dropConfigCache();
 }