made the filesystem loader compatible with Twig 2.0

Without adding the exists() method, the code happens to work by chance,
just because the current implementation of Twig exits() method calls
findTemplate().

But we know that it won't be the case anymore as of Twig 2.0.

src/Symfony/Bundle/TwigBundle/Loader/FilesystemLoader.php

@@ -40,6 +40,25 @@ public function __construct(FileLocatorInterface $locator, TemplateNameParserInt
         $this->cache = array();
     }
 
+    /**
+     * {@inheritdoc}
+     */
+    public function exists($template)
+    {
+        if (parent::exists($template)) {
+            return true;
+        }
+
+        // same logic as findTemplate below for the fallback
+        try {
+            $this->cache[(string) $template] = $this->locator->locate($this->parser->parse($template));
+        } catch (\Exception $e) {
+            return false;
+        }
+
+        return true;
+    }
+
     /**
      * Returns the path to the template file.
      *

src/Symfony/Bundle/TwigBundle/Tests/Loader/FilesystemLoaderTest.php

@@ -38,6 +38,21 @@ public function testGetSource()
         $this->assertEquals("This is a layout\n", $loader->getSource('TwigBundle::layout.html.twig'));
     }
 
+    public function testExists()
+    {
+        // should return true for templates that Twig does not find, but Symfony does
+        $parser = $this->getMock('Symfony\Component\Templating\TemplateNameParserInterface');
+        $locator = $this->getMock('Symfony\Component\Config\FileLocatorInterface');
+        $locator
+            ->expects($this->once())
+            ->method('locate')
+            ->will($this->returnValue($template = __DIR__.'/../DependencyInjection/Fixtures/Resources/views/layout.html.twig'))
+        ;
+        $loader = new FilesystemLoader($locator, $parser);
+
+        return $this->assertTrue($loader->exists($template));
+    }
+
     /**
      * @expectedException Twig_Error_Loader
      */