substr function
Implementation
String substr(String str, int start, int len) {
assert(len >= 0);
if (len == 0) return '';
start = start > 0 ? start : str.length + start;
start = min(start, str.length - 1);
var end = start + len;
end = min(end, str.length - 1);
return str.substring(start, end);
}