7 files declare their use of FileSystem

BuildTestBase.php in core/tests/Drupal/BuildTests/Framework/BuildTestBase.php
DirectoryTest.php in core/tests/Drupal/KernelTests/Core/File/DirectoryTest.php
FileSystem.php in core/lib/Drupal/Core/File/FileSystem.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

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