Write test coverage for views_fetch_fields

+++ b/lib/Drupal/views/Tests/ViewsDataTest.phpundefined
@@ -49,4 +49,67 @@ class ViewsDataTest extends ViewTestBase {
+      $this->assertEqual($expected_keys, array_keys($fields), format_string('Make sure the handlers of type @handler_type are listed as expected.', array('@handler_type' => $handler_type)));

I would maybe change this to just say "Handlers of type @handler_type are listed as expected"

Otherwise, this look good.