fake-fetch-event.js

return new Promise((resolve, reject) => {
  const event = new FetchEvent('fetch', {
    request: new Request('/index.html'),
  });
  event.respondWith = (promiseChain) => {
    if (promiseChain) {
      // Check if promise was returned - otherise
      // it could be a response
      if (promiseChain instanceof Promise) {
        promiseChain.then(resolve, reject);
      } else {
        resolve(promiseChain);
      }
      return;
    }

    resolve();
  };
  self.dispatchEvent(event);
})