WARN] /Users/benjyw/.cache/pants/setup/bootstrap-Darwin-x86_64/pants.DBRM4r/install/lib/python2.7/site-packages/pants/engine/legacy/structs.py:63: DeprecationWarning: DEPRECATED: default empty sources list will be removed in version 1.5.0.dev0.
  Targets which do not explicitly pass sources will soon get a default set. Please pass an explicit set of sources for target: <snip>
  sources = self.get_sources()