private static function cache_wp_files() {
		self::$cache_dir = sys_get_temp_dir() . '/wp-cli-test core-download-cache';

		if ( is_readable( self::$cache_dir . '/wp-config-sample.php' ) )
			return;

		$cmd = Utils\esc_cmd( 'wp core download --force --path=%s', self::$cache_dir );
		Process::create( $cmd, null, self::get_process_env_variables() )->run_check();
	}