[FrameworkBundle] Register an identity translator as fallback

The Form component can be used without the Translation component.
However, to be able to use the default form themes provided by the
FrameworkBundle you need to have the `translator` helper to be
available.

This change ensure that there will always be a `translator` helper which
as a fallback will just return the message key if no translator is
present.

Author: yceruto

Extension/TranslationExtension.php

@@ -17,6 +17,7 @@
 use Symfony\Bridge\Twig\TokenParser\TransDefaultDomainTokenParser;
 use Symfony\Bridge\Twig\TokenParser\TransTokenParser;
 use Symfony\Contracts\Translation\TranslatorInterface;
+use Symfony\Contracts\Translation\TranslatorTrait;
 use Twig\Extension\AbstractExtension;
 use Twig\NodeVisitor\NodeVisitorInterface;
 use Twig\TokenParser\AbstractTokenParser;
@@ -29,6 +30,13 @@
  */
 class TranslationExtension extends AbstractExtension
 {
+    use TranslatorTrait {
+        getLocale as private;
+        setLocale as private;
+        trans as private doTrans;
+        transChoice as private doTransChoice;
+    }
+
     private $translator;
     private $translationNodeVisitor;
 
@@ -91,7 +99,7 @@ public function getTranslationNodeVisitor()
     public function trans($message, array $arguments = array(), $domain = null, $locale = null)
     {
         if (null === $this->translator) {
-            return strtr($message, $arguments);
+            return $this->doTrans($message, $arguments, $domain, $locale);
         }
 
         return $this->translator->trans($message, $arguments, $domain, $locale);
@@ -100,7 +108,7 @@ public function trans($message, array $arguments = array(), $domain = null, $loc
     public function transchoice($message, $count, array $arguments = array(), $domain = null, $locale = null)
     {
         if (null === $this->translator) {
-            return strtr($message, $arguments);
+            return $this->doTransChoice($message, $count, array_merge(array('%count%' => $count), $arguments), $domain, $locale);
         }
 
         return $this->translator->transChoice($message, $count, array_merge(array('%count%' => $count), $arguments), $domain, $locale);