Support form types that don't have a service (#244)

Form types don't have to have a corresponding service tagged as form.type.
If they have no dependencies, the FQCN can be instantiated directly.

This is especially important as most core form types are no longer
registered as services since Symfony 3.1.

Form/Extension/DependencyInjectionExtension.php

@@ -90,7 +90,11 @@ public function getType($name)
         $name = self::findClass($this->mappingTypes, $name);
 
         if (!isset($this->typeServiceIds[$name])) {
-            throw new InvalidArgumentException(sprintf('The field type "%s" is not registered with the service container.', $name));
+            if (class_exists($name) && in_array('Symfony\Component\Form\FormTypeInterface', class_implements($name), true)) {
+                return new $name();
+            } else {
+                throw new InvalidArgumentException(sprintf('The field type "%s" is not registered with the service container.', $name));
+            }
         }
 
         $type = $this->container->get($this->typeServiceIds[$name]);

Tests/Form/Extension/DependencyInjectionExtensionTest.php

@@ -50,6 +50,15 @@ public function testValidType()
         $f->getType('Symfony\Component\Form\Type\FormType');
     }
 
+    public function testTypeWithoutService()
+    {
+        $container = $this->getMock('Symfony\Component\DependencyInjection\ContainerInterface');
+
+        $f = new DependencyInjectionExtension($container, array(), array(), array(), array());
+
+        $this->assertInstanceOf('Symfony\Component\Form\Extension\Core\Type\HiddenType', $f->getType('Symfony\Component\Form\Extension\Core\Type\HiddenType'));
+    }
+
     public function testTypeExtensionsValid()
     {
         $container = $this->getMock('Symfony\Component\DependencyInjection\ContainerInterface');