code: def diff_lines(self):
      
      start_lines = self._build_file_source_lines[:]
      end_lines = self.build_file_lines()
      diff_generator = unified_diff(start_lines,
                                    end_lines,
                                    fromfile=self.build_file.relpath,
                                    tofile=self.build_file.relpath,
                                    lineterm='')
      return list(diff_generator)
docs: A diff between the original BUILD file and the resulting BUILD file.