/**
 * Waits for a load (or custom) event to finish in a given tab. If provided
 * load an uri into the tab.
 *
 * @param tab
 *        The tab to load into.
 * @param [optional] url
 *        The url to load, or the current url.
 * @return {Promise} resolved when the event is handled.
 * @resolves to the received event
 * @rejects if a valid load event is not received within a meaningful interval
 */
function promiseTabLoadEvent(tab, url) {
  info("Wait tab event: load");

  function handle(loadedUrl) {
    if (loadedUrl === "about:blank" || (url && loadedUrl !== url)) {
      info(`Skipping spurious load event for ${loadedUrl}`);
      return false;
    }

    info("Tab event received: load");
    return true;
  }

  let loaded = BrowserTestUtils.browserLoaded(tab.linkedBrowser, false, handle);

  if (url)
    BrowserTestUtils.loadURI(tab.linkedBrowser, url);

  return loaded;
}