Enable native_constant_invocation CS fixer

Author: nicolas-grekas

Filesystem.php

@@ -170,7 +170,7 @@ public function remove($files)
         foreach ($files as $file) {
             if (is_link($file)) {
                 // See https://bugs.php.net/52176
-                if (!(self::box('unlink', $file) || '\\' !== DIRECTORY_SEPARATOR || self::box('rmdir', $file)) && file_exists($file)) {
+                if (!(self::box('unlink', $file) || '\\' !== \DIRECTORY_SEPARATOR || self::box('rmdir', $file)) && file_exists($file)) {
                     throw new IOException(sprintf('Failed to remove symlink "%s": %s.', $file, self::$lastError));
                 }
             } elseif (is_dir($file)) {
@@ -321,7 +321,7 @@ private function isReadable($filename)
      */
     public function symlink($originDir, $targetDir, $copyOnWindows = false)
     {
-        if ('\\' === DIRECTORY_SEPARATOR) {
+        if ('\\' === \DIRECTORY_SEPARATOR) {
             $originDir = strtr($originDir, '/', '\\');
             $targetDir = strtr($targetDir, '/', '\\');
 
@@ -343,7 +343,7 @@ public function symlink($originDir, $targetDir, $copyOnWindows = false)
 
         if (!self::box('symlink', $originDir, $targetDir)) {
             if (null !== self::$lastError) {
-                if ('\\' === DIRECTORY_SEPARATOR && false !== strpos(self::$lastError, 'error code(1314)')) {
+                if ('\\' === \DIRECTORY_SEPARATOR && false !== strpos(self::$lastError, 'error code(1314)')) {
                     throw new IOException('Unable to create symlink due to error code 1314: \'A required privilege is not held by the client\'. Do you have the required Administrator-rights?', 0, null, $targetDir);
                 }
             }
@@ -362,7 +362,7 @@ public function symlink($originDir, $targetDir, $copyOnWindows = false)
     public function makePathRelative($endPath, $startPath)
     {
         // Normalize separators on Windows
-        if ('\\' === DIRECTORY_SEPARATOR) {
+        if ('\\' === \DIRECTORY_SEPARATOR) {
             $endPath = str_replace('\\', '/', $endPath);
             $startPath = str_replace('\\', '/', $startPath);
         }