core#1643 - Prevent broken paths on windows during civicrm-setup
On windows the vendor/composer/autoload_civicrm_asset.php file gets created with paths like:
$civicrm_paths['civicrm.root']['path'] = $baseDir . '/C:\\path\\to\\vendor/civicrm/civicrm-core/';
This fixes it so it doesn't do that. After the patch it's possible to install Drupal 8 on windows using cv core:install (although you still at the moment need the hack mentioned in the first part of the ticket in CRM/Core/CodeGen/Util/Xml.php).