19 files declare their use of EntityReferenceItem

DefaultSelection.php in core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php
EntityReferenceFieldItemNormalizer.php in core/modules/serialization/src/Normalizer/EntityReferenceFieldItemNormalizer.php
EntityReferenceFieldItemNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/EntityReferenceFieldItemNormalizerTest.php
EntityReferenceFieldItemNormalizerTrait.php in core/modules/serialization/src/Normalizer/EntityReferenceFieldItemNormalizerTrait.php
EntityReferenceFormatterBase.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php
EntityReferenceItemSubclass.php in core/modules/media_library/tests/modules/media_library_test/src/Plugin/Field/FieldType/EntityReferenceItemSubclass.php
EntityReferenceUuidItem.php in core/modules/jsonapi/tests/modules/jsonapi_test_field_type/src/Plugin/Field/FieldType/EntityReferenceUuidItem.php
EntityResourceTestBase.php in core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
FieldConfigEditForm.php in core/modules/field_ui/src/Form/FieldConfigEditForm.php
FileFormatterBase.php in core/modules/file/src/Plugin/Field/FieldFormatter/FileFormatterBase.php
FileItem.php in core/modules/file/src/Plugin/Field/FieldType/FileItem.php
MediaLibraryHooks.php in core/modules/media_library/src/Hook/MediaLibraryHooks.php
MediaThumbnailFormatter.php in core/modules/media/src/Plugin/Field/FieldFormatter/MediaThumbnailFormatter.php
NodeFieldHooks.php in core/modules/node/src/Hook/NodeFieldHooks.php
ResourceIdentifier.php in core/modules/jsonapi/src/JsonApiResource/ResourceIdentifier.php
TaxonomyFieldHooks.php in core/modules/taxonomy/src/Hook/TaxonomyFieldHooks.php
TimestampItemNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/TimestampItemNormalizerTest.php
UserFieldHooks.php in core/modules/user/src/Hook/UserFieldHooks.php
XmlEntityNormalizationQuirksTrait.php in core/modules/rest/tests/src/Functional/EntityResource/XmlEntityNormalizationQuirksTrait.php

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