loadExternalDtd method
Attempts to load the external DTD file from externalDtdUri and
parse the DTD elements contained within. A new XmlDoctype
will
be returned with the parsed DTD elements in the externalDtd value.
If the URI couldn't be reached or no DTD elements could be identified,
Returns null
.
Implementation
Future<XmlDoctype?> loadExternalDtd() async {
if (externalDtdUri == null) return null;
final externalEntities = await XmlNode.fromUri(externalDtdUri!);
if (externalEntities == null) return null;
return copyWith(externalDtd: externalEntities);
}