how to add border radius to google maps in flutter?

Keep in mind that the google_maps_flutter plugin is only a developer preview. I'm sure a lot more work will be added to this before they release version 1.0. So don't stress too much about missing features. File tickets. :)

probably an expensive solution but you could use ClipRRect. Something like this:

Widget build(BuildContext context) {
    return Center(
      child: ClipRRect(
        borderRadius: BorderRadius.only(
          topLeft: Radius.circular(30),
          topRight: Radius.circular(30),
          bottomRight: Radius.circular(30),
          bottomLeft: Radius.circular(30),
        ),
        child: Align(
          alignment: Alignment.bottomRight,
          heightFactor: 0.3,
          widthFactor: 2.5,
          child: GoogleMap(),
        ),
      ),
    );
  }