chromium/third_party/blink/web_tests/http/tests/devtools/service-workers/resources/network-fetch-worker.js

self.onmessage = (event) => {
  fetch(new Request(event.data.url, event.data.init)).then(
      (response) => {
        return response.text().then((text) => event.source.postMessage(text));
      },
      () => event.source.postMessage('FETCH_FAILED'));
};