docs: def backend(self):
      "Internal property that returns the Node script running harness"
      if self._backend is None:
        with io.open(path.join(path.abspath(path.dirname(__file__)), "runner.js")) as runner_file:
          runner_source = runner_file.read()
        self._backend = execjs.ExternalRuntime(name="Node.js (V8)",
                                               command=["node"],
                                               runner_source=runner_source)
      return self._backend