Fix the configurability of CoreExtension deps in standalone usage

When using the Forms entrypoint to configure the component, there was
no chance to configure dependencies of the CoreExtension, as the one
registered without argument was first and would win.
The builder now delays the prepending of the CoreExtension to do it only
if the CoreExtension is not registered explicitly.

src/Symfony/Component/Form/FormFactoryBuilder.php

@@ -11,13 +11,17 @@
 
 namespace Symfony\Component\Form;
 
+use Symfony\Component\Form\Extension\Core\CoreExtension;
+
 /**
  * The default implementation of FormFactoryBuilderInterface.
  *
  * @author Bernhard Schussek <bschussek@gmail.com>
  */
 class FormFactoryBuilder implements FormFactoryBuilderInterface
 {
+    private $forceCoreExtension;
+
     /**
      * @var ResolvedFormTypeFactoryInterface
      */
@@ -43,6 +47,14 @@ class FormFactoryBuilder implements FormFactoryBuilderInterface
      */
     private $typeGuessers = [];
 
+    /**
+     * @param bool $forceCoreExtension
+     */
+    public function __construct($forceCoreExtension = false)
+    {
+        $this->forceCoreExtension = $forceCoreExtension;
+    }
+
     /**
      * {@inheritdoc}
      */
@@ -144,6 +156,21 @@ public function getFormFactory()
     {
         $extensions = $this->extensions;
 
+        if ($this->forceCoreExtension) {
+            $hasCoreExtension = false;
+
+            foreach ($extensions as $extension) {
+                if ($extension instanceof CoreExtension) {
+                    $hasCoreExtension = true;
+                    break;
+                }
+            }
+
+            if (!$hasCoreExtension) {
+                array_unshift($extensions, new CoreExtension());
+            }
+        }
+
         if (\count($this->types) > 0 || \count($this->typeExtensions) > 0 || \count($this->typeGuessers) > 0) {
             if (\count($this->typeGuessers) > 1) {
                 $typeGuesser = new FormTypeGuesserChain($this->typeGuessers);

src/Symfony/Component/Form/Forms.php

@@ -105,10 +105,7 @@ public static function createFormFactory()
      */
     public static function createFormFactoryBuilder()
     {
-        $builder = new FormFactoryBuilder();
-        $builder->addExtension(new CoreExtension());
-
-        return $builder;
+        return new FormFactoryBuilder(true);
     }
 
     /**