7 files declare their use of DataDefinitionInterface in Zircon Profile 8

DateTimeComputed.php in core/modules/datetime/src/DateTimeComputed.php

Contains \Drupal\datetime\DateTimeComputed.

EntityFieldTest.php in core/modules/system/src/Tests/Entity/EntityFieldTest.php

Contains \Drupal\system\Tests\Entity\EntityFieldTest.

FieldItemBase.php in core/lib/Drupal/Core/Field/FieldItemBase.php

Contains \Drupal\Core\Field\FieldItemBase.

FieldTestItem.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/FieldTestItem.php

Contains \Drupal\entity_test\Plugin\Field\FieldType\FieldTestItem.

ListElement.php in core/modules/config_translation/src/FormElement/ListElement.php

Contains \Drupal\config_translation\FormElement\ListElement.

TextProcessed.php in core/modules/text/src/TextProcessed.php

Contains \Drupal\text\TextProcessed.

TypedDataDefinitionTest.php in core/modules/system/src/Tests/TypedData/TypedDataDefinitionTest.php

Contains \Drupal\system\Tests\TypedData\TypedDataDefinitionTest.