descape method
Implementation
String descape(int pos, String s) {
int i = 0;
int x = 0;
while (i < 4) {
final n = _HexChars[s.codeUnitAt(i)];
if (n < 0) {
die(pos, 'expected valid unicode escape');
}
x = (x << 4) | n;
i += 1;
}
return String.fromCharCode(x);
}