List<VarSymbol> freevars(ClassSymbol c)  {
        if ((c.owner.kind & (VAR | MTH)) != 0) {
            List<VarSymbol> fvs = freevarCache.get(c);
            if (fvs == null) {
                FreeVarCollector collector = new FreeVarCollector(c);
                collector.scan(classDef(c));
                fvs = collector.fvs;
                freevarCache.put(c, fvs);
            }
            return fvs;
        } else {
            return List.nil();
        }
    }