substring method
Implementation
String substring(int start, [int? end]) {
end ??= _length;
if (start < 0 || end < start || end > _length) {
throw RangeError('Invalid range: [$start, $end) for length $_length');
}
if (start == end) return '';
if (_root == null) return '';
final buffer = StringBuffer();
for (final chunk in _root!.chunksInRange(start, end)) {
buffer.write(chunk);
}
return buffer.toString();
}