9 files declare their use of PluginDefinition

AnnotationBridgeDecoratorTest.php in core/tests/Drupal/Tests/Component/Annotation/Plugin/Discovery/AnnotationBridgeDecoratorTest.php
CKEditor5PluginDefinition.php in core/modules/ckeditor5/src/Plugin/CKEditor5PluginDefinition.php
ContextAwarePluginBaseTest.php in core/tests/Drupal/KernelTests/Core/Plugin/Context/ContextAwarePluginBaseTest.php
ContextAwarePluginTraitTest.php in core/tests/Drupal/KernelTests/Core/Plugin/Context/ContextAwarePluginTraitTest.php
ContextHandlerTest.php in core/tests/Drupal/Tests/Core/Plugin/ContextHandlerTest.php
Contains \Drupal\Tests\Core\Plugin\ContextHandlerTest.
DefaultPluginManagerTest.php in core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
EntityType.php in core/lib/Drupal/Core/Entity/EntityType.php
LayoutDefinition.php in core/lib/Drupal/Core/Layout/LayoutDefinition.php
SectionStorageDefinition.php in core/modules/layout_builder/src/SectionStorage/SectionStorageDefinition.php

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