11 files declare their use of HtmlCommand in Drupal 8

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

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

AjaxTestImageEffect.php in core/modules/image/tests/modules/image_module_test/src/Plugin/ImageEffect/AjaxTestImageEffect.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.

Callbacks.php in core/modules/system/tests/modules/ajax_forms_test/src/Callbacks.php

ContentModerationConfigureEntityTypesForm.php in core/modules/content_moderation/src/Form/ContentModerationConfigureEntityTypesForm.php

EditorImageDialog.php in core/modules/editor/src/Form/EditorImageDialog.php

EditorLinkDialog.php in core/modules/editor/src/Form/EditorLinkDialog.php

EditorMediaDialog.php in core/modules/media/src/Form/EditorMediaDialog.php

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

ViewEditForm.php in core/modules/views_ui/src/ViewEditForm.php