When verifying the libraries/ckeditor/plugins/dialogadvtab/plugin.js file exists, use the Drupal root directory path

The code that verifies the libraries/ckeditor/plugins/dialogadvtab/plugin.js exists should add the Drupal root directory path. This is already done from the hook_requirements implementation, but it's not done from the plugin class, which should still return a path relative to the Drupal root directory, in getFile().

Instead of DRUPAL_ROOT, the code should use the app.root container parameter. See app.root and site.path services have been converted to container parameters for how this can be implemented in code that needs to be compatible with Drupal 8 and Drupal 9.