[google_maps_flutter_web] Calculate marker anchor from image size if available

Uses the size info from the image configuration if available to convert the anchor from Flutter API unit coordinates to JS API pixels.

flutter/flutter#80578

Author: AsturaPhoenix

packages/google_maps_flutter/google_maps_flutter_web/lib/src/convert.dart

@@ -244,7 +244,8 @@ gmaps.Size? _gmSizeFromIconConfig(List<Object?> iconConfig, int sizeIndex) {
 }
 
 // Converts a [BitmapDescriptor] into a [gmaps.Icon] that can be used in Markers.
-gmaps.Icon? _gmIconFromBitmapDescriptor(BitmapDescriptor bitmapDescriptor) {
+gmaps.Icon? _gmIconFromBitmapDescriptor(
+    BitmapDescriptor bitmapDescriptor, Offset anchor) {
   final List<Object?> iconConfig = bitmapDescriptor.toJson() as List<Object?>;
 
   gmaps.Icon? icon;
@@ -279,7 +280,11 @@ gmaps.Icon? _gmIconFromBitmapDescriptor(BitmapDescriptor bitmapDescriptor) {
     }
   }
 
-  return icon;
+  return icon
+    ?..anchor = icon.size == null
+        ? null
+        : gmaps.Point(
+            anchor.dx * icon.size!.width!, anchor.dy * icon.size!.height!);
 }
 
 // Computes the options for a new [gmaps.Marker] from an incoming set of options
@@ -298,8 +303,7 @@ gmaps.MarkerOptions _markerOptionsFromMarker(
     ..visible = marker.visible
     ..opacity = marker.alpha
     ..draggable = marker.draggable
-    ..icon = _gmIconFromBitmapDescriptor(marker.icon);
-  // TODO(ditman): Compute anchor properly, otherwise infowindows attach to the wrong spot.
+    ..icon = _gmIconFromBitmapDescriptor(marker.icon, marker.anchor);
   // Flat and Rotation are not supported directly on the web.
 }