in lib/src/codepage.dart [394:401]
static Map<int, int> _createBmpEncoding(String characters) {
var encoding = <int, int>{};
for (var i = 0; i < characters.length; i++) {
var char = characters.codeUnitAt(i);
if (char != 0xFFFD) encoding[characters.codeUnitAt(i)] = i;
}
return encoding;
}