12 files declare their use of BaseFieldDefinition

BaseFieldOverride.php in core/lib/Drupal/Core/Field/Entity/BaseFieldOverride.php
ContentEntityBase.php in core/lib/Drupal/Core/Entity/ContentEntityBase.php
entity.api.php in core/lib/Drupal/Core/Entity/entity.api.php
Hooks and documentation related to entities.
EntityDefinitionUpdateManager.php in core/lib/Drupal/Core/Entity/EntityDefinitionUpdateManager.php
EntityDefinitionUpdateProviderTest.php in core/tests/Drupal/KernelTests/Core/Entity/EntityDefinitionUpdateProviderTest.php
EntityFieldManager.php in core/lib/Drupal/Core/Entity/EntityFieldManager.php
EntityPublishedTrait.php in core/lib/Drupal/Core/Entity/EntityPublishedTrait.php
EntityTestComputedBundleField.php in core/modules/system/tests/modules/entity_test/src/Entity/EntityTestComputedBundleField.php
EntityTestUniqueConstraint.php in core/modules/system/tests/modules/entity_test/src/Entity/EntityTestUniqueConstraint.php
FieldItemDataDefinition.php in core/lib/Drupal/Core/Field/TypedData/FieldItemDataDefinition.php
RevisionLogEntityTrait.php in core/lib/Drupal/Core/Entity/RevisionLogEntityTrait.php
SqlContentEntityStorageSchema.php in core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php

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