class CustomBlock

1. 8.x drupal/core/modules/block/custom_block/lib/Drupal/custom_block/Plugin/Derivative/CustomBlock.php \Drupal\custom_block\Plugin\Derivative\CustomBlock
2. 8.x drupal/core/modules/block/custom_block/lib/Drupal/custom_block/Plugin/Core/Entity/CustomBlock.php \Drupal\custom_block\Plugin\Core\Entity\CustomBlock

class CustomBlock implements DerivativeInterface {

  /**
   * List of derivative definitions.
   *
   * @var array
   */
  protected $derivatives = array();

  /**
   * Implements \Drupal\Component\Plugin\Derivative\DerivativeInterface::getDerivativeDefinition().
   *
   * Retrieves a specific custom block definition from storage.
   */
  public function getDerivativeDefinition($derivative_id, array $base_plugin_definition) {
    if (!empty($this->derivatives) && !empty($this->derivatives[$derivative_id])) {
      return $this->derivatives[$derivative_id];
    }
    $this
      ->getDerivativeDefinitions($base_plugin_definition);
    return $this->derivatives[$derivative_id];
  }

  /**
   * Implements \Drupal\Component\Plugin\Derivative\DerivativeInterface::getDerivativeDefinitions().
   *
   * Retrieves custom block definitions from storage.
   */
  public function getDerivativeDefinitions(array $base_plugin_definition) {
    $custom_blocks = entity_load_multiple('custom_block');
    foreach ($custom_blocks as $custom_block) {
      $this->derivatives[$custom_block->uuid->value] = $base_plugin_definition;
      $this->derivatives[$custom_block->uuid->value]['admin_label'] = $custom_block->info->value;
    }
    return $this->derivatives;
  }

}