static proxyEvent_(event) {
    const eventType = event.type.replace(/^(webkit|moz|MS)/, '').toLowerCase();

    const newEvent = document.createEvent('Event');
    newEvent.initEvent(eventType, event.bubbles, event.cancelable);

    event.target.dispatchEvent(newEvent);
  }
Proxy fullscreen events after changing their name.
@param {!Event} event
@private