subspan method
Returns a span from start
code units (inclusive) to end
code units
(exclusive) after the beginning of this span.
Implementation
SourceSpan subspan(int start, [int? end]) {
RangeError.checkValidRange(start, end, length);
if (start == 0 && (end == null || end == length)) return this;
final locations = subspanLocations(this, start, end);
return SourceSpan(locations[0], locations[1], text.substring(start, end));
}