9 files declare their use of FileSystem in Drupal 8

BuildTestBase.php in core/tests/Drupal/BuildTests/Framework/BuildTestBase.php

DirectoryTest.php in core/tests/Drupal/KernelTests/Core/File/DirectoryTest.php

file.inc in core/includes/file.inc

API for handling file uploads and server file management.

FileSystem.php in core/lib/Drupal/Core/File/FileSystem.php

FileSystemRequirementsTest.php in core/tests/Drupal/KernelTests/Core/File/FileSystemRequirementsTest.php

FileSystemTempDirectoryTest.php in core/tests/Drupal/KernelTests/Core/File/FileSystemTempDirectoryTest.php

system.install in core/modules/system/system.install

Install, update and uninstall functions for the system module.

TestDatabase.php in core/lib/Drupal/Core/Test/TestDatabase.php

TestSiteApplicationTest.php in core/tests/Drupal/Tests/Scripts/TestSiteApplicationTest.php