Correct testing error.

core/lib/Drupal/Component/PhpStorage/FileStorage.php

@@ -94,7 +94,7 @@ protected function ensureDirectory($directory, $mode = 0777) {
    */
   protected function createDirectory($directory, $mode = 0777) {
     // If running under Windows replace illegal characters with _.
-    $directory = $this->sanitizeFilename($directory);
+    $directory = Drupal\Component\PhpStorage\MTimeProtectedFastFileStorage::sanitizeFilename($directory);
     // If the directory exists already, there's nothing to do.
     if (is_dir($directory)) {
       return TRUE;