fromBase64 method
Decodes the string from Base64 encoding and returns the resulting string.
If decoding fails, the method returns a default value.
def is the default value returned in case of a decoding error.
Returns the decoded string if successful; otherwise, returns def.
Implementation
String fromBase64({String def = ''}) {
  try {
    List<int> bytes = base64.decode(this);
    return utf8.decode(bytes);
  } catch (e) {
    return def;
  }
}