8 files declare their use of AttachedAssetsInterface in Drupal 9

AssetResolverTest.php in core/tests/Drupal/Tests/Core/Asset/AssetResolverTest.php

Contains \Drupal\Tests\Core\Asset\AssetResolverTest.

BigPipe.php in core/modules/big_pipe/src/Render/BigPipe.php

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

Helper module for the Common tests.

HtmlResponseAttachmentsProcessor.php in core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php

locale.module in core/modules/locale/locale.module

Enables the translation of the user interface to languages other than English.

settings_tray.module in core/modules/settings_tray/settings_tray.module

Allows configuring blocks and other configuration from the site front-end.

system.module in core/modules/system/system.module

Configuration system that lets administrators modify the workings of the site.

user.module in core/modules/user/user.module

Enables the user registration and login system.