anchor.diff:  /// Encodes lists of bytes using base64 or base64url encoding.
   ///
   /// The results are ASCII strings using a restricted alphabet.
  +///
  +/// Example:
  +/// ```dart
  +/// final base64Encoder = base64.encoder;
  +/// const sample = 'Dart is open source';
  +/// final encodedSample = base64Encoder.convert(sample.codeUnits);
  +/// print(encodedSample); // RGFydCBpcyBvcGVuIHNvdXJjZQ==
  +/// ```
   class Base64Encoder extends Converter<List<int>, String> {
     final bool _urlSafe;
   
  
positive.code:   /// 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
    /// will raise an error anyway, and we only care about being precise for
    /// successful conversions.
    static int _trimPaddingChars(String input, int start, int end) {
  
negative.code:   /// only call this function after having seen at least one `=` or `%`
    /// character.
    /// If the number of missing characters is not 3 or 0, we have seen (at least)
    /// a `%` character and expect the rest of the `%3D` sequence, and a `=` is
    /// not allowed. When missing 3 characters, either `=` or `%` is allowed.
    ///
    /// When the value is 0, no more padding (or any other character) is allowed.
  