17 files declare their use of PoItem in Drupal 8

common.inc in core/includes/common.inc

Common functions that many Drupal modules will need to reference.

locale.module in core/modules/locale/locale.module

Enables the translation of the user interface to languages other than English.

LocaleJavascriptTranslationTest.php in core/modules/locale/tests/src/Functional/LocaleJavascriptTranslationTest.php

LocaleLocaleLookupTest.php in core/modules/locale/tests/src/Functional/LocaleLocaleLookupTest.php

LocaleLookup.php in core/modules/locale/src/LocaleLookup.php

LocaleLookupTest.php in core/modules/locale/tests/src/Unit/LocaleLookupTest.php

LocalePluralFormatTest.php in core/modules/locale/tests/src/Functional/LocalePluralFormatTest.php

NumericField.php in core/modules/views/src/Plugin/views/field/NumericField.php

NumericFormatPluralTest.php in core/modules/views/tests/src/Functional/Plugin/NumericFormatPluralTest.php

PluralTranslatableMarkup.php in core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php

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

PoDatabaseReader.php in core/modules/locale/src/PoDatabaseReader.php

PoDatabaseWriter.php in core/modules/locale/src/PoDatabaseWriter.php

PoStreamWriterTest.php in core/tests/Drupal/Tests/Component/Gettext/PoStreamWriterTest.php

StringBase.php in core/modules/locale/src/StringBase.php

system.module in core/modules/system/system.module

Configuration system that lets administrators modify the workings of the site.

TranslateEditForm.php in core/modules/locale/src/Form/TranslateEditForm.php