Merge branch '4.2'

* 4.2:
  bump required Twig version
  fix compatibility with Twig >= 2.6.1
  [Form] SA fix
  fix compatibility with PHPUnit 4.8
  remove return type hint for PHP 5 compatibility
  SCA: minor code tweaks
  Component CssSelector tests
  [DebugClassLoader] Readd findFile() method
  [Console] Fix composer.json suggest/provide
  Revert "bug symfony#29597 [DI] fix reporting bindings on overriden services as unused (nicolas-grekas)"
  Fixed exception wording
  Fix SwiftMailerHandler to support Monolog's latest reset functionality

Mapping/Loader/XmlFileLoader.php

@@ -75,7 +75,8 @@ public function loadClassMetadata(ClassMetadataInterface $classMetadata)
             if (isset($xml->{'discriminator-map'})) {
                 $mapping = array();
                 foreach ($xml->{'discriminator-map'}->mapping as $element) {
-                    $mapping[(string) $element->attributes()->type] = (string) $element->attributes()->class;
+                    $elementAttributes = $element->attributes();
+                    $mapping[(string) $elementAttributes->type] = (string) $elementAttributes->class;
                 }
 
                 $classMetadata->setClassDiscriminatorMapping(new ClassDiscriminatorMapping(