12 files declare their use of DataDefinitionInterface in Drupal 8

ComputedFileUrlTest.php in core/modules/file/tests/src/Kernel/ComputedFileUrlTest.php

ContentEntityNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/ContentEntityNormalizerTest.php

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

EntityFieldTest.php in core/tests/Drupal/KernelTests/Core/Entity/EntityFieldTest.php

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

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

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

InternalTypedDataTestTrait.php in core/modules/serialization/tests/src/Unit/Normalizer/InternalTypedDataTestTrait.php

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

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

TimestampItemNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php

TypedDataDefinitionTest.php in core/tests/Drupal/KernelTests/Core/TypedData/TypedDataDefinitionTest.php