maps sample - add widget on top of map

main.dart

body: Stack(
 children: <Widget>[
   GoogleMap(
     onMapCreated: _onMapCreated,
     options: GoogleMapOptions(
       cameraPosition: CameraPosition(
         target: _center,
         zoom: 11.0,
       ),
     ),
   ),
   Padding(
     padding: const EdgeInsets.all(16.0),
     child: Align(
       alignment: Alignment.topRight,
       child: FloatingActionButton(
         onPressed: () => print('button pressed'),
         materialTapTargetSize: MaterialTapTargetSize.padded,
         backgroundColor: Colors.green,
         child: const Icon(Icons.map, size: 36.0),
       ),
     ),
   ),
 ],
)