bug #1874 Fix adding mock extension to Twig environment (GromNaN)

This PR was merged into the 1.x branch.

Discussion
----------

Fix adding mock extension to Twig environment

When the extension class is not defined in a file but in eval'd code we cannot assume `ReflectionObject::getFileName()` will return a valid file path.
https://bugs.php.net/bug.php?id=63901

This PR adds a test case that produce the following error without the fix:

```
1) Twig_Tests_EnvironmentTest::testAddMockExtension
filemtime(): stat failed for phar:///usr/local/php5-5.5.14-20140628-105310/bin/phpunit/phpunit-mock-objects/Framework/MockObject/Generator.php(335) : eval()'d code

/xxx/Twig/vendor/symfony/phpunit-bridge/DeprecationErrorHandler.php:40
/xxx/Twig/lib/Twig/Environment.php:769
/xxx/Twig/test/Twig/Tests/EnvironmentTest.php:304
```

I encountered this issue with mocked twig extensions in SonataAdminBundle test suite:
https://github.com/sonata-project/SonataAdminBundle/blob/master/Tests/Controller/HelperControllerTest.php#L254

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1870
| License       | MIT
| Doc PR        | n/a

Commits
-------

a57b1cc Fix adding mock extension to Twig environment

lib/Twig/Environment.php

@@ -462,7 +462,7 @@ class Twig_Environment
         if (0 === $this->lastModifiedExtension) {
             foreach ($this->extensions as $extension) {
                 $r = new ReflectionObject($extension);
-                if (($extensionTime = filemtime($r->getFileName())) > $this->lastModifiedExtension) {
+                if (file_exists($r->getFileName()) && ($extensionTime = filemtime($r->getFileName())) > $this->lastModifiedExtension) {
                     $this->lastModifiedExtension = $extensionTime;
                 }
             }

test/Twig/Tests/EnvironmentTest.php

@@ -292,6 +292,22 @@ class Twig_Tests_EnvironmentTest extends PHPUnit_Framework_TestCase
         $this->assertCount(2, $twig->getNodeVisitors());
     }
 
+    public function testAddMockExtension()
+    {
+        $extension = $this->getMock('Twig_ExtensionInterface');
+        $extension->expects($this->once())
+            ->method('getName')
+            ->will($this->returnValue('mock'));
+
+        $loader = new Twig_Loader_Array(array('page' => 'hey'));
+
+        $twig = new Twig_Environment($loader);
+        $twig->addExtension($extension);
+
+        $this->assertInstanceOf('Twig_ExtensionInterface', $twig->getExtension('mock'));
+        $this->assertTrue($twig->isTemplateFresh('page', time()));
+    }
+
     protected function getMockLoader($templateName, $templateContent)
     {
         $loader = $this->getMock('Twig_LoaderInterface');