plugin-8.x-2.8/tests/modules/plugin_test_helper/src/Plugin/PluginTestHelper/MockPlugin.php

<?php

namespace Drupal\plugin_test_helper\Plugin\PluginTestHelper;

use Drupal\Core\Plugin\PluginBase;

/**
 * Provides a mock plugin.
 */
class MockPlugin extends PluginBase implements MockPluginInterface {
}