code: function parseExtension(extension) {
    var extensionSplit = %_Call(StringSplit, extension, '-');
  
    // Assume ['', 'u', ...] input, but don't throw.
    if (extensionSplit.length <= 2 ||
        (extensionSplit[0] !== '' && extensionSplit[1] !== 'u')) {
      return {};
    }
  
    // Key is {2}alphanum, value is {3,8}alphanum.
    // Some keys may not have explicit values (booleans).
    var extensionMap = {};
    var previousKey = UNDEFINED;
    for (var i = 2; i < extensionSplit.length; ++i) {
      var length = extensionSplit[i].length;
      var element = extensionSplit[i];
      if (length === 2) {
        extensionMap[element] = UNDEFINED;
        previousKey = element;
      } else if (length >= 3 && length <=8 && !IS_UNDEFINED(previousKey)) {
        extensionMap[previousKey] = element;
        previousKey = UNDEFINED;
      } else {
        // There is a value that's too long, or that doesn't have a key.
        return {};
      }
    }
  
    return extensionMap;
  }
docs: Parses Unicode extension into key - value map.
  Returns empty object if the extension string is invalid.
  We are not concerned with the validity of the values at this point.