consider AccNo on FAnon to be read-only

Every untyped field access causes these and it becomes a mess if we temp var these. I'm not entirely sure if this is correct...

src/optimization/optimizer.ml

@@ -247,6 +247,7 @@ let is_read_only_field_access fa = match fa with
 		match cf.cf_kind with
 			| Method MethDynamic -> false
 			| Method _ -> true
+			| Var {v_write = AccNo} when (match fa with FAnon _ -> true | _ -> false) -> true
 			| Var {v_write = AccNever} -> true
 			| _ -> false