15 files declare their use of FieldItemList

BaseFieldOverrideTest.php in core/tests/Drupal/KernelTests/Core/Field/Entity/BaseFieldOverrideTest.php
CommentFieldItemList.php in core/modules/comment/src/CommentFieldItemList.php
ComputedTestBundleFieldItemList.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/ComputedTestBundleFieldItemList.php
ComputedTestCacheableIntegerItemList.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/ComputedTestCacheableIntegerItemList.php
ComputedTestCacheableStringItemList.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/ComputedTestCacheableStringItemList.php
ComputedTestFieldItemList.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/ComputedTestFieldItemList.php
DateRangeFieldItemList.php in core/modules/datetime_range/src/Plugin/Field/FieldType/DateRangeFieldItemList.php
DateTimeFieldItemList.php in core/modules/datetime/src/Plugin/Field/FieldType/DateTimeFieldItemList.php
FieldItem.php in core/lib/Drupal/Core/Field/Plugin/DataType/FieldItem.php
FieldItemListTest.php in core/tests/Drupal/Tests/Core/Field/FieldItemListTest.php
LayoutSectionItemList.php in core/modules/layout_builder/src/Field/LayoutSectionItemList.php
LinkFormatterTest.php in core/modules/link/tests/src/Unit/LinkFormatterTest.php
ModerationStateFieldItemList.php in core/modules/content_moderation/src/Plugin/Field/ModerationStateFieldItemList.php
PathFieldItemList.php in core/modules/path/src/Plugin/Field/FieldType/PathFieldItemList.php
TextFieldItemList.php in core/modules/text/src/Plugin/Field/FieldType/TextFieldItemList.php

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