Problems with anonymous methods and the unit JS

If I add the unit JS to a project, it seems that it enables generally enables the objfpc mode.

Here is a test project, that compiles

program Project1;

//uses
//  JS;

type
  TRouteProc = reference to procedure; async;

  TFoo = class
  public
   class procedure AddRoute(Proc: TRouteProc); overload;
  end;

{ TFoo }

class procedure TFoo.AddRoute(Proc: TRouteProc);
begin
end;

procedure Test; async;
begin
end;

begin
  TFoo.AddRoute(Test);
end.

If I add the unit JS to the uses clause, the project does not compile any longer. It seems that then the objfpc mode is enabled.