code: function SingleByteDecoder(index, options) {
      var fatal = options.fatal;
      /**
       * @param {Stream} stream The stream of bytes being decoded.
       * @param {number} bite The next byte read from the stream.
       * @return {?(number|!Array.<number>)} The next code point(s)
       *     decoded, or null if not enough data exists in the input
       *     stream to decode a complete code point.
       */
      this.handler = function(stream, bite) {
        // 1. If byte is end-of-stream, return finished.
        if (bite === end_of_stream)
          return finished;
  
        // 2. If byte is an ASCII byte, return a code point whose value
        // is byte.
        if (isASCIIByte(bite))
          return bite;
  
        // 3. Let code point be the index code point for byte − 0x80 in
        // index single-byte.
        var code_point = index[bite - 0x80];
  
        // 4. If code point is null, return error.
        if (code_point === null)
          return decoderError(fatal);
  
        // 5. Return a code point whose value is code point.
        return code_point;
      };
    }
docs: 10.1 single-byte decoder 
  @constructor
  @implements {Decoder}
  @param {!Array.<number>} index The encoding index.
  @param {{fatal: boolean}} options