6 files declare their use of FileStorage in Zircon Profile 8

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

Contains \Drupal\Core\Composer\Composer.

DirectoryTest.php in core/modules/system/src/Tests/File/DirectoryTest.php

Contains \Drupal\system\Tests\File\DirectoryTest.

file.inc in core/includes/file.inc

API for handling file uploads and server file management.

FileSaveHtaccessLoggingTest.php in core/modules/system/src/Tests/File/FileSaveHtaccessLoggingTest.php

Contains \Drupal\system\Tests\File\FileSaveHtaccessLoggingTest.

FileStorageReadOnlyTest.php in core/tests/Drupal/Tests/Component/PhpStorage/FileStorageReadOnlyTest.php

Contains \Drupal\Tests\Component\PhpStorage\FileStorageReadOnlyTest.

FileStorageTest.php in core/tests/Drupal/Tests/Component/PhpStorage/FileStorageTest.php

Contains \Drupal\Tests\Component\PhpStorage\FileStorageTest.