public static Map<String, UrlNode> asUrlNodeMap(Map<String, UrlReference> urlReferenceMap) {
        return Collections.unmodifiableMap(
                    urlReferenceMap.entrySet().stream().collect(
                        Collectors.toMap(Map.Entry::getKey, e -> new UrlNode(e.getValue()))
                    ));
    }
Special case for file type, since FileNode param type (FileReference) is not same as type (String) in config builder
config-lib/src/main/java/com/yahoo/config/LeafNodeMaps.java