Merge branch '2.7' into 2.8

* 2.7:
  [FrameworkBundle] Check for class existence before is_subclass_of
  Update GroupSequence.php
  Code enhancement and cleanup
  [DI] Add anti-regression test
  Revert "minor #19689 [DI] Cleanup array_key_exists (ro0NL)"
  [BrowserKit] Fix cookie expiration on 32 bit systems
  bumped Symfony version to 2.7.18
  updated VERSION for 2.7.17
  update CONTRIBUTORS for 2.7.17
  updated CHANGELOG for 2.7.17
  Update misleading comment about RFC4627

Author: nicolas-grekas

Dumper/MoFileDumper.php

@@ -36,7 +36,7 @@ public function format(MessageCatalogue $messages, $domain = 'messages')
      */
     public function formatCatalogue(MessageCatalogue $messages, $domain, array $options = array())
     {
-        $output = $sources = $targets = $sourceOffsets = $targetOffsets = '';
+        $sources = $targets = $sourceOffsets = $targetOffsets = '';
         $offsets = array();
         $size = 0;
 

Loader/MoFileLoader.php

@@ -80,7 +80,7 @@ protected function loadResource($resource)
         $messages = array();
 
         for ($i = 0; $i < $count; ++$i) {
-            $singularId = $pluralId = null;
+            $pluralId = null;
             $translated = null;
 
             fseek($stream, $offsetId + $i * 8);