gecko-dev/netwerk/test/unit_ipc/test_alt-data_simple_wrap.js

const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");

// needs to be rooted
var cacheFlushObserver = {
  observe() {
    cacheFlushObserver = null;
    do_send_remote_message("flushed");
  },
};

function run_test() {
  do_get_profile();
  do_await_remote_message("flush").then(() => {
    Services.cache2
      .QueryInterface(Ci.nsICacheTesting)
      .flush(cacheFlushObserver);
  });
  run_test_in_child("../unit/test_alt-data_simple.js");
}