gistfile1.js

(function() {
  var requestAutocomplete = HTMLFormElement.prototype.requestAutocomplete;
  HTMLFormElement.prototype.requestAutocomplete = function() {
     var form = this;
     return new Promise(function(resolve, reject) {
         requestAutocomplete.call(form);
         function cleanup() {
            form.removeEventListener(onSuccess);
            form.removeEventListener(onError);
         }
         function onSuccess() {
            resolve();
            cleanup();
         }
         function onError(event) {
            reject(event.reason);
            cleanup();
         }
         form.addEventListener("autocomplete", onSuccess);
         form.addEventListener("autocompleteerror", onError);
     });
  };
})();

Rejection reasons should always be Error instances.