Change record status: 
Project: 
Introduced in branch: 
8.0.x
Description: 

File system functions are now available as methods on the 'file_system' service, and are found in \Drupal\Core\File\FileSystem

Before:
drupal_chmod($uri);
After:
\Drupal::service('file_system')->chmod($uri)

Old function New method
drupal_move_uploaded_file() \Drupal\Core\File\FileSystem::moveUploadedFile()
drupal_chmod() \Drupal\Core\File\FileSystem::chmod()
drupal_unlink() \Drupal\Core\File\FileSystem::unlink()
drupal_realpath() \Drupal\Core\File\FileSystem::realpath()
drupal_dirname() \Drupal\Core\File\FileSystem::dirname()
drupal_basename() \Drupal\Core\File\FileSystem::basename()
drupal_mkdir() \Drupal\Core\File\FileSystem::mkdir()
drupal_rmdir() \Drupal\Core\File\FileSystem::rmdir()
drupal_tempnam() \Drupal\Core\File\FileSystem::tempnam()
file_htaccess_lines() \Drupal\Component\PhpStorage\FileStorage::htaccessLines()

Additionally, the constants FILE_CHMOD_DIRECTORY and FILE_CHMOD_FILE were deprecated in favor of \Drupal\Core\File\FileSystem::CHMOD_DIRECTORY and \Drupal\Core\File\FileSystem::CHMOD_FILE.

The following functions have been deprecated and moved to StreamWrapperManager

Old function/method New method
FileSystem::uriScheme()
file_uri_scheme()
StreamWrapperManager::getScheme()
FileSystem::validScheme()
file_stream_wrapper_valid_scheme()
\Drupal::service('stream_wrapper_manager')->isValidScheme()
Impacts: 
Module developers
Updates Done (doc team, etc.)
Online documentation: 
Not done
Theming guide: 
Not done
Module developer documentation: 
Not done
Examples project: 
Not done
Coder Review: 
Not done
Coder Upgrade: 
Not done
Other: 
Other updates done