Handle events emitted on browser.

def event_handler(msg: EventMsgDict) -> Event:
    """Handle events emitted on browser."""
    e = create_event_from_msg(msg)
    if e.currentTarget is None:
        if e.type not in ['mount', 'unmount']:
            id = msg['currentTarget']['id']
            logger.warning('No such element: wdom_id={}'.format(id))
        return e
    e.currentTarget.on_event_pre(e)
    e.currentTarget.dispatchEvent(e)
    return e