Merge branch '2.8' into 3.0

* 2.8:
  improved comment
  [FileSystem] Add support for Google App Engine.
  [2.8] fix mocks
  [Form] Fix BC break introduced in #14403

Author: fabpot

Resources/views/Form/widget_attributes.html.php

@@ -1,6 +1,8 @@
-id="<?php echo $view->escape($id) ?>" name="<?php echo $view->escape($full_name) ?>"<?php if ($disabled): ?> disabled="disabled"<?php endif ?>
+id="<?php echo $view->escape($id) ?>" name="<?php echo $view->escape($full_name) ?>"<?php if ($read_only): ?> readonly="readonly"<?php endif ?>
+<?php if ($disabled): ?> disabled="disabled"<?php endif ?>
 <?php if ($required): ?> required="required"<?php endif ?>
 <?php foreach ($attr as $k => $v): ?>
+<?php if ('readonly' === $k) { continue; } ?>
 <?php if (in_array($k, array('placeholder', 'title'), true)): ?>
 <?php printf(' %s="%s"', $view->escape($k), $view->escape(false !== $translation_domain ? $view['translator']->trans($v, array(), $translation_domain) : $v)) ?>
 <?php elseif ($v === true): ?>