6 files declare their use of AlertCommand in Drupal 8

AjaxCommandsTest.php in core/tests/Drupal/Tests/Core/Ajax/AjaxCommandsTest.php

AjaxRenderer.php in core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php

AjaxTestController.php in core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php

ajax_forms_test.module in core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module

Simpletest mock module for Ajax forms testing.

FormAjaxResponseBuilderTest.php in core/tests/Drupal/Tests/Core/Form/FormAjaxResponseBuilderTest.php

FrameworkTest.php in core/modules/system/tests/src/Functional/Ajax/FrameworkTest.php