     return key;
    }

read: async (key) => {
      let link = document.createElement("link");
      link.rel = "prefetch";
      link.href = testURI("resource", "prefetch", key);
      document.getElementsByTagName("head")[0].appendChild(link);
      await sleepMs(500);
      let response = await fetch(
        testURI("ctr", "prefetch", key), {"cache": "reload"});
      let countString = (await response.text()).trim();
      if (parseInt(countString) === 0) {
        throw new Error("No requests received