added a BC comment

src/Symfony/Component/Config/Loader/FileLoader.php

@@ -67,6 +67,9 @@ abstract class FileLoader extends Loader
             $loader = $this->resolve($resource, $type);
 
             if ($loader instanceof FileLoader && null !== $this->currentDir) {
+                // we fallback to the current locator to keep BC
+                // as some some loaders do not call the parent __construct()
+                // @deprecated should be removed in 3.0
                 $locator = $loader->getLocator() ?: $this->locator;
                 $resource = $locator->locate($resource, $this->currentDir, false);
             }