createWorld(transport, ...args) {
    const worldId = this.worldId_ + '/' + (++this.lastWorldId_);
    const sendToChild = transport(this.routeMessage_.bind(this, false));
    this.worlds_.set(worldId, sendToChild);
    sendToChild({cookie: true, args: this.wrap_(args), worldId });
    return new Promise(f => this.cookieResponseCallbacks_.set(worldId, f));
  }
Called in the parent world.
Creates a child world with the given root handle.

@param {!Transport} transport
       - receives function that should be called upon messages from
         the world and
       - returns function that should be used to send messages to the
         world
@param {...*} args Params to pass to the child world.
@return {!Promise<{worldId:string, *}>} returns the handles / parameters that child
        world returned during the initialization.