function createEventPromise(target, eventType) {
  return new Promise(function(resolve, reject) {
    events.listen(target, eventType, resolve);
  });
}
Returns promise that resolves when eventType is dispatched from target.
@param {!EventTarget|!events.Listenable} target Target to listen for
    event on.
@param {string} eventType Type of event.
@return {!Promise} Promise that resolves with triggered event.