23 files declare their use of Inspector in Drupal 8

ApcuBackend.php in core/lib/Drupal/Core/Cache/ApcuBackend.php

Cache.php in core/lib/Drupal/Core/Cache/Cache.php

CacheableNormalization.php in core/modules/jsonapi/src/Normalizer/Value/CacheableNormalization.php

CacheCollector.php in core/lib/Drupal/Core/Cache/CacheCollector.php

CacheTagsInvalidator.php in core/lib/Drupal/Core/Cache/CacheTagsInvalidator.php

Data.php in core/modules/jsonapi/src/JsonApiResource/Data.php

DatabaseBackend.php in core/lib/Drupal/Core/Cache/DatabaseBackend.php

DefaultPluginManager.php in core/lib/Drupal/Core/Plugin/DefaultPluginManager.php

EntityResource.php in core/modules/jsonapi/src/Controller/EntityResource.php

EntityResourceTestBase.php in core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php

ErrorCollection.php in core/modules/jsonapi/src/JsonApiResource/ErrorCollection.php

IncludedData.php in core/modules/jsonapi/src/JsonApiResource/IncludedData.php

InspectorTest.php in core/tests/Drupal/Tests/Component/Assertion/InspectorTest.php

Contains \Drupal\Tests\Component\Assertion\InspectorTest.

Link.php in core/modules/jsonapi/src/JsonApiResource/Link.php

LinkCollection.php in core/modules/jsonapi/src/JsonApiResource/LinkCollection.php

MemoryBackend.php in core/lib/Drupal/Core/Cache/MemoryBackend.php

MemoryCache.php in core/lib/Drupal/Core/Cache/MemoryCache/MemoryCache.php

OmittedData.php in core/modules/jsonapi/src/JsonApiResource/OmittedData.php

PhpBackend.php in core/lib/Drupal/Core/Cache/PhpBackend.php

RelationshipData.php in core/modules/jsonapi/src/JsonApiResource/RelationshipData.php

ResourceObjectData.php in core/modules/jsonapi/src/JsonApiResource/ResourceObjectData.php

ResourceTypeBuildEvent.php in core/modules/jsonapi/src/ResourceType/ResourceTypeBuildEvent.php

ResourceTypeRepository.php in core/modules/jsonapi/src/ResourceType/ResourceTypeRepository.php