Merge remote-tracking branch 'origin/2.7' into 2.8

* origin/2.7:
  symfony#24046 added check for ext-dom to XmlUtil::loadFile

Author: fabpot

src/Symfony/Component/Config/Util/XmlUtils.php

@@ -37,9 +37,14 @@ private function __construct()
      * @return \DOMDocument
      *
      * @throws \InvalidArgumentException When loading of XML file returns error
+     * @throws \RuntimeException         When DOM extension is missing
      */
     public static function loadFile($file, $schemaOrCallable = null)
     {
+        if (!extension_loaded('dom')) {
+            throw new \RuntimeException('Extension DOM is required.');
+        }
+
         $content = @file_get_contents($file);
         if ('' === trim($content)) {
             throw new \InvalidArgumentException(sprintf('File %s does not contain valid XML, it is empty.', $file));