Implement support for file: includes in OWNERS files.

testing_support/filesystem_mock.py

Index: testing_support/filesystem_mock.py
diff --git a/testing_support/filesystem_mock.py b/testing_support/filesystem_mock.py
index 07e1834ffe14aecbfab2441c20db51eb4077e47b..b45ea8e37409a766009e1afbb4e10b0dad357a3b 100644
--- a/testing_support/filesystem_mock.py
+++ b/testing_support/filesystem_mock.py
@@ -38,6 +38,11 @@ class MockFileSystem(object):
       return path[:-1]
     return path
 
+  def basename(self, path):
+    if self.sep not in path:
+      return ''
+    return self._split(path)[-1] or self.sep
+
   def dirname(self, path):
     if self.sep not in path:
       return ''