31 files declare their use of BlockBase

AggregatorFeedBlock.php in core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php
AjaxFormBlock.php in core/modules/system/tests/modules/ajax_forms_test/src/Plugin/Block/AjaxFormBlock.php
AttachedRenderingBlock.php in core/modules/system/tests/modules/render_attached_test/src/Plugin/Block/AttachedRenderingBlock.php
BlockContentBlock.php in core/modules/block_content/src/Plugin/Block/BlockContentBlock.php
BlockFormTest.php in core/modules/block/tests/src/Unit/BlockFormTest.php
HelpBlock.php in core/modules/help/src/Plugin/Block/HelpBlock.php
LocalActionsBlock.php in core/lib/Drupal/Core/Menu/Plugin/Block/LocalActionsBlock.php
LocalTasksBlock.php in core/lib/Drupal/Core/Menu/Plugin/Block/LocalTasksBlock.php
NodeContextTestBlock.php in core/modules/node/tests/modules/node_block_test/src/Plugin/Block/NodeContextTestBlock.php
PageTitleBlock.php in core/lib/Drupal/Core/Block/Plugin/Block/PageTitleBlock.php
RedirectFormBlock.php in core/modules/system/tests/modules/form_test/src/Plugin/Block/RedirectFormBlock.php
SearchBlock.php in core/modules/search/src/Plugin/Block/SearchBlock.php
SyndicateBlock.php in core/modules/node/src/Plugin/Block/SyndicateBlock.php
SystemBrandingBlock.php in core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
SystemBreadcrumbBlock.php in core/modules/system/src/Plugin/Block/SystemBreadcrumbBlock.php
SystemMainBlock.php in core/modules/system/src/Plugin/Block/SystemMainBlock.php
SystemMenuBlock.php in core/modules/system/src/Plugin/Block/SystemMenuBlock.php
SystemMessagesBlock.php in core/modules/system/src/Plugin/Block/SystemMessagesBlock.php
SystemPoweredByBlock.php in core/modules/system/src/Plugin/Block/SystemPoweredByBlock.php
TestAccessBlock.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestAccessBlock.php
TestBlockInstantiation.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestBlockInstantiation.php
TestCacheBlock.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestCacheBlock.php
TestContextAwareBlock.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareBlock.php
TestContextAwareNoValidContextOptionsBlock.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareNoValidContextOptionsBlock.php
TestContextAwareUnsatisfiedBlock.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareUnsatisfiedBlock.php
TestFormBlock.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestFormBlock.php
TestHtmlBlock.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestHtmlBlock.php
TestMultipleFormsBlock.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php
TestSettingsValidationBlock.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestSettingsValidationBlock.php
UserLoginBlock.php in core/modules/user/src/Plugin/Block/UserLoginBlock.php
ViewsBlockBase.php in core/modules/views/src/Plugin/Block/ViewsBlockBase.php

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