6 files declare their use of IntegerData in Drupal 8

ConfigEntityAdapterTest.php in core/tests/Drupal/KernelTests/Core/Entity/ConfigEntityAdapterTest.php

DateTimeIso8601NormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/DateTimeIso8601NormalizerTest.php

DateTimeNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/DateTimeNormalizerTest.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

TimestampNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/TimestampNormalizerTest.php