addCharsUntilMultipleOfFour function
Add as much equals signs at the end of the string to have a length that is multiple of four
Implementation
String addCharsUntilMultipleOfFour(String inputBase64) {
if (inputBase64.length % 4 == 0) return inputBase64;
return addCharsUntilMultipleOfFour('$inputBase64=');
}