def depend_on(path)
      if environment.absolute_path?(path) && environment.stat(path)
        @dependencies << environment.build_file_digest_uri(path)
      else
        resolve(path)
      end
      nil
    end
def depend_on(path)
      if environment.absolute_path?(path) && environment.stat(path)
        @dependencies << environment.build_file_digest_uri(path)
      else
        resolve(path)
      end
      nil
    end
`depend_on` allows you to state a dependency on a file without
 including it.

 This is used for caching purposes. Any changes made to
 the dependency file will invalidate the cache of the
 source file.