anchor.diff:    /// it needs when input is valid, and at least enough bytes to reach the error
     /// when input is invalid.
     ///
  -  /// Never count more than two padding sequences since any more than that
  +  /// Never count more than two padding sequences as any more than that
     /// will raise an error anyway, and we only care about being precise for
     /// successful conversions.
     static int _trimPaddingChars(String input, int start, int end) {
  
positive.code: /// Encodes lists of bytes using base64 or base64url encoding.
  ///
  /// The results are ASCII strings using a restricted alphabet.
  class Base64Encoder extends Converter<List<int>, String> {
    final bool _urlSafe;
  
  
negative.code: /// This decoder accepts both base64 and base64url ("url-safe") encodings.
  ///
  /// The encoding is required to be properly padded.
  ///
  /// Throws a [FormatException] if the input is not valid base64 data.
  ///
  /// Example:
  /// ```dart
  /// final base64Decoder = base64.decoder;
  /// const base64Bytes = 'RGFydCBpcyBvcGVuIHNvdXJjZQ==';
  /// final decodedBytes = base64Decoder.convert(base64Bytes);
  /// // decodedBytes: [68, 97, 114, 116, 32, 105, 115, 32, 111, 112, 101, 110,
  /// // 32, 115, 111, 117, 114, 99, 101]
  ///
  /// // Print as string using UTF-8 decoder
  /// print(utf8.decode(decodedBytes)); // Dart is open source
  /// ```
  class Base64Decoder extends Converter<String, List<int>> {
    const Base64Decoder();
  
  