6 files declare their use of Callbacks

AjaxFormsTestAjaxElementsForm.php in core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestAjaxElementsForm.php
AjaxFormsTestCommandsForm.php in core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestCommandsForm.php
AjaxFormsTestImageButtonForm.php in core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestImageButtonForm.php
AjaxFormsTestLazyLoadForm.php in core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestLazyLoadForm.php
AjaxFormsTestSimpleForm.php in core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestSimpleForm.php
AjaxFormsTestValidationForm.php in core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestValidationForm.php

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