10 files declare their use of Routes

DefaultExceptionSubscriber.php in core/modules/jsonapi/src/EventSubscriber/DefaultExceptionSubscriber.php
EntityReferenceFieldNormalizer.php in core/modules/jsonapi/src/Normalizer/EntityReferenceFieldNormalizer.php
EntityUuidConverter.php in core/modules/jsonapi/src/ParamConverter/EntityUuidConverter.php
jsonapi.module in core/modules/jsonapi/jsonapi.module
Module implementation file.
Relationship.php in core/modules/jsonapi/src/JsonApiResource/Relationship.php
RelationshipRouteAccessCheck.php in core/modules/jsonapi/src/Access/RelationshipRouteAccessCheck.php
ResourceObject.php in core/modules/jsonapi/src/JsonApiResource/ResourceObject.php
ResourceResponseValidatorTest.php in core/modules/jsonapi/tests/src/Unit/EventSubscriber/ResourceResponseValidatorTest.php
ResourceVersionRouteEnhancer.php in core/modules/jsonapi/src/Revisions/ResourceVersionRouteEnhancer.php
RoutesTest.php in core/modules/jsonapi/tests/src/Unit/Routing/RoutesTest.php

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