function getByte(default_val) {
    while (nextCharIndex < input.length) {
      var ch = input.charAt(nextCharIndex++);
      var b = goog.crypt.base64.charToByteMap_[ch];
      if (b != null) {
        return b;  // Common case: decoded the char.
      }
      if (!goog.string.internal.isEmptyOrWhitespace(ch)) {
        throw new Error('Unknown base64 encoding at char: ' + ch);
      }
      // We encountered whitespace: loop around to the next input char.
    }
    return default_val;  // No more input remaining.
  }
@param {number} default_val Used for end-of-input.
@return {number} The next 6-bit value, or the default for end-of-input.