Changelog:

Drupal_get_path() and drupal_get_filename() have been deprecated. Use an extension type-specific  \Drupal\Core\Extension\ExtensionList::getPath() and \Drupal\Core\Extension\ExtensionList::getPathname() instead where possible. You can use \Drupal::service('extension.path.resolver')->getPath()/getPathname() and dynamically pass in the extension type as the first parameter.

Before:

drupal_get_path('theme', 'MYTHEME') . '/css/main.css';

After:

\Drupal::service('extension.list.theme')->getPath('MYTHEME')