Bindings generator often uses string-based type refs instead of referencing a wrapper

[Zverik]

Often for a package with multiple classes, I find in declarations something like this:

              BridgeTypeAnnotation(
                BridgeTypeRef(
                  BridgeTypeSpec(
                    'package:http/src/base_request.dart',
                    'BaseRequest',
                  ),
                  [],
                ),
                nullable: true,
              ),

instead of this:

              BridgeTypeAnnotation($BaseRequest.$type, nullable: true),

I don't know how hard it is to keep a map of all wrapped classes, but it would be handy not to have to replace those after a conversion.