anchor.diff:    /// 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 expects the rest of the `%3D` sequence, and a `=` is
  +  /// 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.
  
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:   /// Returns a [Uint8List] of the ASCII codes of the encoded data.
    ///
    /// If the input, including left over [_state] from earlier encodings,
    /// is not a multiple of three bytes, then the partial state is stored
    /// back into [_state].
    /// If [isLast] is true, partial state is encoded in the output instead,
    /// with the necessary padding.
  