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chromium/third_party/blink/web_tests/external/wpt/service-workers/service-worker/navigation-preload/resources/empty-preload-response-body-worker.js 

self.addEventListener('activate', event => {
    event.waitUntil(
        self.registration.navigationPreload.enable());
  });

self.addEventListener('fetch', event => {
    event.respondWith(
      event.preloadResponse
        .then(res => res.text())
        .then(text => {
            return new Response(
                '<body>[' + text + ']</body>',
                {headers: [['content-type', 'text/html']]});
          }));
  });