8 files declare their use of StringData

ConfigEntityAdapterTest.php in core/tests/Drupal/KernelTests/Core/Entity/ConfigEntityAdapterTest.php
ConfigSchemaTest.php in core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php
ContextAwarePluginBaseTest.php in core/tests/Drupal/KernelTests/Core/Plugin/Context/ContextAwarePluginBaseTest.php
ContextTypedDataTest.php in core/tests/Drupal/KernelTests/Core/Plugin/ContextTypedDataTest.php
FilterFormat.php in core/modules/filter/src/Plugin/DataType/FilterFormat.php
PrimitiveDataNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/PrimitiveDataNormalizerTest.php
PrimitiveTypeConstraintValidatorTest.php in core/tests/Drupal/Tests/Core/Validation/Plugin/Validation/Constraint/PrimitiveTypeConstraintValidatorTest.php
StringNormalizer.php in core/modules/jsonapi/tests/modules/jsonapi_test_data_type/src/Normalizer/StringNormalizer.php

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