chromium/third_party/blink/web_tests/external/wpt/workers/support/throw-on-message-Worker.js

self.onerror = function(evt) {
  postMessage('error');
  return true;
}

self.onmessage = function(evt) {
    if (evt.data === "first")
        throw Error();
    else
        postMessage(evt.data);
}