Assert that (legacy) marker's icon is not a PinConfig

Author: aednlaxer

packages/google_maps_flutter/google_maps_flutter_platform_interface/lib/src/types/marker.dart

@@ -160,7 +160,11 @@ class Marker implements MapsObject<Marker> {
     this.onDrag,
     this.onDragStart,
     this.onDragEnd,
-  }) : assert(0.0 <= alpha && alpha <= 1.0);
+  })  : assert(0.0 <= alpha && alpha <= 1.0),
+        assert(
+          icon is! PinConfig,
+          'Icon must not be a PinConfig. PinConfig is only supported by AdvancedMarker.',
+        );
 
   /// Uniquely identifies a [Marker].
   final MarkerId markerId;