load method
Loads the next document from the stream.
If there are no more documents, returns null.
Implementation
YamlDocument? load() {
if (_parser.isDone) return null;
var event = _parser.parse();
if (event.type == EventType.streamEnd) {
_span = _span.expand(event.span);
return null;
}
var document = _loadDocument(event as DocumentStartEvent);
_span = _span.expand(document.span as FileSpan);
_aliases.clear();
return document;
}