15 files declare their use of BlockInterface

AnnounceBlockTest.php in core/modules/announcements_feed/tests/src/FunctionalJavascript/AnnounceBlockTest.php
Block.php in core/modules/block/src/Entity/Block.php
block.post_update.php in core/modules/block/block.post_update.php
Post update functions for Block.
BlockController.php in core/modules/block/src/Controller/BlockController.php
BlockEntitySettingTrayForm.php in core/modules/settings_tray/src/Block/BlockEntitySettingTrayForm.php
BlockHasOverridesAccessCheck.php in core/modules/settings_tray/src/Access/BlockHasOverridesAccessCheck.php
BlockPluginHasSettingsTrayFormAccessCheck.php in core/modules/settings_tray/src/Access/BlockPluginHasSettingsTrayFormAccessCheck.php
BlockPluginHasSettingsTrayFormAccessCheckTest.php in core/modules/settings_tray/tests/src/Unit/Access/BlockPluginHasSettingsTrayFormAccessCheckTest.php
block_content.module in core/modules/block_content/block_content.module
Allows the creation of content blocks through the user interface.
menu_ui.module in core/modules/menu_ui/menu_ui.module
Allows administrators to customize the site's navigation menus.
PlaceBlock.php in core/modules/block/src/Plugin/ConfigAction/PlaceBlock.php
search.module in core/modules/search/search.module
Enables site-wide keyword searching.
search.post_update.php in core/modules/search/search.post_update.php
Post update functions for Search module.
settings_tray.module in core/modules/settings_tray/settings_tray.module
Allows configuring blocks and other configuration from the site front-end.
views_ui.module in core/modules/views_ui/views_ui.module
Provide structure for the administrative interface to Views.

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