edlp_d8/core/modules/contextual/tests/modules/contextual_test/contextual_test.module

<?php

/**
 * @file
 * Provides test contextual link on blocks.
 */

use Drupal\Core\Block\BlockPluginInterface;

/**
 * Implements hook_block_view_alter().
 */
function contextual_test_block_view_alter(array &$build, BlockPluginInterface $block) {
  $build['#contextual_links']['contextual_test'] = [
    'route_parameters' => [],
  ];
}

/**
 * Implements hook_contextual_links_view_alter().
 *
 * @todo Apparently this too late to attach the library?
 * It won't work without contextual_test_page_attachments_alter()
 * Is that a problem? Should the contextual module itself do the attaching?
 */
function contextual_test_contextual_links_view_alter(&$element, $items) {
  if (isset($element['#links']['contextual-test-ajax'])) {
    $element['#attached']['library'][] = 'core/drupal.dialog.ajax';
  }
}

/**
 * Implements hook_page_attachments_alter().
 */
function contextual_test_page_attachments_alter(array &$attachments) {
  $attachments['#attached']['library'][] = 'core/drupal.dialog.ajax';
}