13 files declare their use of LinkItemInterface in Drupal 8

LinkFieldTest.php in core/modules/link/tests/src/Functional/LinkFieldTest.php

LinkFieldUITest.php in core/modules/link/tests/src/Functional/LinkFieldUITest.php

LinkFormatter.php in core/modules/link/src/Plugin/Field/FieldFormatter/LinkFormatter.php

LinkItem.php in core/modules/link/src/Plugin/Field/FieldType/LinkItem.php

LinkItemSerializationTest.php in core/modules/link/tests/src/Kernel/LinkItemSerializationTest.php

LinkItemTest.php in core/modules/link/tests/src/Kernel/LinkItemTest.php

LinkTypeConstraintValidator.php in core/modules/link/src/Plugin/Validation/Constraint/LinkTypeConstraintValidator.php

LinkWidget.php in core/modules/link/src/Plugin/Field/FieldWidget/LinkWidget.php

link_test_base_field.module in core/modules/link/tests/modules/link_test_base_field/link_test_base_field.module

Contains main module functions.

MenuLinkContent.php in core/modules/menu_link_content/src/Entity/MenuLinkContent.php

MigrateFieldInstanceTest.php in core/modules/field/tests/src/Kernel/Migrate/d6/MigrateFieldInstanceTest.php

RouteCachingLanguageTest.php in core/tests/Drupal/FunctionalTests/Routing/RouteCachingLanguageTest.php

Shortcut.php in core/modules/shortcut/src/Entity/Shortcut.php