[autotest] Make empty batch retriever subclass

BUG=chromium:672348
TEST=None

Change-Id: Id5b984f33c29e1784c60db32d361ce7b5dd928f1
Reviewed-on: https://chromium-review.googlesource.com/453264
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/server/cros/dynamic_suite/suite.py b/server/cros/dynamic_suite/suite.py
index 92a63ea..bebbcaf 100644
--- a/server/cros/dynamic_suite/suite.py
+++ b/server/cros/dynamic_suite/suite.py
@@ -571,6 +571,10 @@
             yield path, self._cf_getter.get_control_file_contents(path)
 
 
+class _BatchControlFileRetriever(_ControlFileRetriever):
+    """Subclass that can retrieve suite control files in batch."""
+
+
 def _parse_control_file_texts(control_file_texts,
                               forgiving_parser=True, run_prod_code=False,
                               test_args=None):