16 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.
BlockContentHooks.php in core/modules/block_content/src/Hook/BlockContentHooks.php
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
ClearCacheBlockTest.php in core/modules/system/tests/src/Functional/Block/ClearCacheBlockTest.php
MenuUiHooks.php in core/modules/menu_ui/src/Hook/MenuUiHooks.php
PlaceBlock.php in core/modules/block/src/Plugin/ConfigAction/PlaceBlock.php
search.post_update.php in core/modules/search/search.post_update.php
Post update functions for Search module.
SearchHooks.php in core/modules/search/src/Hook/SearchHooks.php
settings_tray.module in core/modules/settings_tray/settings_tray.module
ViewsUiHooks.php in core/modules/views_ui/src/Hook/ViewsUiHooks.php

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