[dce] do not keep a class if only its extern fields are kept (closes #3546)

dce.ml

@@ -448,7 +448,7 @@ let run com main full =
 			List.iter (fun (c,cf,stat) -> mark_dependent_fields dce c cf.cf_name stat) cfl;
 			(* mark fields as used *)
 			List.iter (fun (c,cf,stat) ->
-				mark_class dce c;
+				if not (is_extern_field cf) then mark_class dce c;
 				mark_field dce c cf stat;
 				mark_t dce cf.cf_pos cf.cf_type
 			) cfl;
@@ -512,8 +512,10 @@ let run com main full =
 				b
 			) c.cl_ordered_fields;
 			(match c.cl_constructor with Some cf when not (keep_field dce cf) -> c.cl_constructor <- None | _ -> ());
+			let inef cf = not (is_extern_field cf) in
+			let has_non_extern_fields = List.exists inef c.cl_ordered_fields || List.exists inef c.cl_ordered_statics in
 			(* we keep a class if it was used or has a used field *)
-			if Meta.has Meta.Used c.cl_meta || c.cl_ordered_statics <> [] || c.cl_ordered_fields <> [] then loop (mt :: acc) l else begin
+			if Meta.has Meta.Used c.cl_meta || has_non_extern_fields then loop (mt :: acc) l else begin
 				(match c.cl_init with
 				| Some f when Meta.has Meta.KeepInit c.cl_meta ->
 					(* it means that we only need the __init__ block *)