[DependencyInjection] changed the YAML notation for optional services from @@ to @?

src/Symfony/Component/DependencyInjection/Dumper/YamlDumper.php

@@ -161,7 +161,7 @@ class YamlDumper extends Dumper
     protected function getServiceCall($id, Reference $reference = null)
     {
         if (null !== $reference && ContainerInterface::EXCEPTION_ON_INVALID_REFERENCE !== $reference->getInvalidBehavior()) {
-            return sprintf('@@%s', $id);
+            return sprintf('@?%s', $id);
         } else {
             return sprintf('@%s', $id);
         }

src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php

@@ -199,7 +199,7 @@ class YamlFileLoader extends FileLoader
     {
         if (is_array($value)) {
             $value = array_map(array($this, 'resolveServices'), $value);
-        } else if (is_string($value) && 0 === strpos($value, '@@')) {
+        } else if (is_string($value) && 0 === strpos($value, '@?')) {
             $value = new Reference(substr($value, 2), ContainerInterface::IGNORE_ON_INVALID_REFERENCE);
         } else if (is_string($value) && 0 === strpos($value, '@')) {
             $value = new Reference(substr($value, 1));

tests/Symfony/Tests/Component/DependencyInjection/Fixtures/yaml/services9.yml

@@ -32,9 +32,9 @@ services:
     file: %path%foo.php
     calls:
       - [setBar, ['@foo']]
-      - [setBar, ['@@foo']]
-      - [setBar, ['@@foo']]
-      - [setBar, ['@@foobaz']]
+      - [setBar, ['@?foo']]
+      - [setBar, ['@?foo']]
+      - [setBar, ['@?foobaz']]
       
   factory_service:
     factory_method: getInstance