public function addFatalErrorHandler($handler) {
        if ( ! $handler instanceof INativeErrorHandler && ! is_callable($handler)) {
            throw new InvalidHandlerType(
                s('Fatal error handler must be a callable or an instance of %s.',
                    INativeErrorHandler::class)
            );
        }

        if (is_callable($handler)) {
            $handler = $this->createNativeErrorHandler($handler);
        }

        $this->fatalErrorHandlers[] = $handler;
    }