getSourcePath example

$connection_options = array_intersect_key($connection_options, $form + $form['advanced_options']);
    // Remove isolation_level since that option is not configurable in the UI.     unset($connection_options['isolation_level']);
    $edit = [
      $driver => $connection_options,
      'version' => '7',
    ];
    if (count($drivers) !== 1) {
      $edit['driver'] = $driver;
    }
    // Set the public and private base paths for the Credential Form.     $edit['source_private_file_path'] = $this->fs->realpath($this->getSourcePath('private'));
    $edit['source_base_path'] = $this->fs->realpath($this->getSourcePath('public'));
    $edits = $this->translatePostValues($edit);

    // Start the upgrade.     $this->drupalGet('/upgrade');
    $this->submitForm([], 'Continue');
    $this->submitForm($edits, 'Review upgrade');

    // The migrations are now in store - remove all but the file migrations.     $store = \Drupal::service('tempstore.private')->get('migrate_drupal_ui');
    $migration_array = array_intersect_key(