13 files declare their use of FieldInterface

EmailItemTest.php in drupal/core/modules/email/lib/Drupal/email/Tests/EmailItemTest.php
Contains \Drupal\email\Tests\EmailItemTest.
EntityFieldTest.php in drupal/core/modules/system/lib/Drupal/system/Tests/Entity/EntityFieldTest.php
Definition of Drupal\system\Tests\Entity\EntityFieldTest.
EntityReferenceItemTest.php in drupal/core/modules/entity_reference/lib/Drupal/entity_reference/Tests/EntityReferenceItemTest.php
Contains \Drupal\entity_reference\Tests\EntityReferenceItemTest.
EntityValidationTest.php in drupal/core/modules/system/lib/Drupal/system/Tests/Entity/EntityValidationTest.php
Contains \Drupal\system\Tests\Entity\EntityValidationTest.
Field.php in drupal/core/lib/Drupal/Core/Entity/Field/Type/Field.php
Contains \Drupal\Core\Entity\Field\Type\Field.
FileItemTest.php in drupal/core/modules/file/lib/Drupal/file/Tests/FileItemTest.php
Contains \Drupal\file\Tests\FileItemTest.
ImageItemTest.php in drupal/core/modules/image/lib/Drupal/image/Tests/ImageItemTest.php
Contains \Drupal\image\Tests\ImageItemTest.
LinkItemTest.php in drupal/core/modules/link/lib/Drupal/link/Tests/LinkItemTest.php
Contains \Drupal\link\Tests\LinkItemTest.
NumberItemTest.php in drupal/core/modules/number/lib/Drupal/number/Tests/NumberItemTest.php
Contains \Drupal\number\Tests\NumberItemTest.
ShapeItemTest.php in drupal/core/modules/field/lib/Drupal/field/Tests/ShapeItemTest.php
Contains \Drupal\field\Tests\ShapeItemTest.
TaxonomyTermReferenceItemTest.php in drupal/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/TaxonomyTermReferenceItemTest.php
Contains \Drupal\taxonomy\Tests\TaxonomyTermReferenceItemTest.
TelephoneItemTest.php in drupal/core/modules/telephone/lib/Drupal/telephone/Tests/TelephoneItemTest.php
Contains \Drupal\field\Tests\TelephoneItemTest.
TestItemTest.php in drupal/core/modules/field/lib/Drupal/field/Tests/TestItemTest.php
Contains \Drupal\field\Tests\TestItemTest.