create_base64_map function
Implementation
List<int> create_base64_map() {
var base64M = List.filled(256, -1);
for (var i = 0; i < base64Chars.length; ++i) {
base64M[base64Chars.codeUnitAt(i)] = i;
}
base64M['='.codeUnitAt(0)] = 0;
return base64M;
}