Update.php does nothing
Drupal 8 requires PHP 5.5.9. If you are using PHP 5.2.9, it doesn't recognize as valid a line like use Drupal\Core\DrupalKernel; since namespaces were introduced in PHP 5.3.

If you were already running a stable release of Drupal 8 before, check you didn't change in someway the PHP version used from your site.