Merge branch '3.1'

* 3.1:
  [PhpUnitBridge] Fix undefined variable
  Compatibility with Twig 1.27
  Remove extra line in doc-block comment
  [VarDumper] Fix dumping Twig source in stack traces
  Enhance GAE compat by removing some realpath()
  [DependencyInjection] Remove old code in XML loader
  bumped Symfony version to 3.1.7
  updated VERSION for 3.1.6
  updated CHANGELOG for 3.1.6
  bumped Symfony version to 2.8.14
  updated VERSION for 2.8.13
  updated CHANGELOG for 2.8.13
  bumped Symfony version to 2.7.21
  updated VERSION for 2.7.20
  update CONTRIBUTORS for 2.7.20
  updated CHANGELOG for 2.7.20
  [SecurityBundle] Fix twig-bridge lowest dep

Author: nicolas-grekas

Iterator/DateRangeFilterIterator.php

@@ -44,7 +44,7 @@ public function accept()
     {
         $fileinfo = $this->current();
 
-        if (!file_exists($fileinfo->getRealPath())) {
+        if (!file_exists($fileinfo->getPathname())) {
             return false;
         }
 

Iterator/SortableIterator.php

@@ -41,7 +41,7 @@ public function __construct(\Traversable $iterator, $sort)
 
         if (self::SORT_BY_NAME === $sort) {
             $this->sort = function ($a, $b) {
-                return strcmp($a->getRealpath(), $b->getRealpath());
+                return strcmp($a->getRealpath() ?: $a->getPathname(), $b->getRealpath() ?: $b->getPathname());
             };
         } elseif (self::SORT_BY_TYPE === $sort) {
             $this->sort = function ($a, $b) {
@@ -51,7 +51,7 @@ public function __construct(\Traversable $iterator, $sort)
                     return 1;
                 }
 
-                return strcmp($a->getRealpath(), $b->getRealpath());
+                return strcmp($a->getRealpath() ?: $a->getPathname(), $b->getRealpath() ?: $b->getPathname());
             };
         } elseif (self::SORT_BY_ACCESSED_TIME === $sort) {
             $this->sort = function ($a, $b) {