patternBasedJoinTwoStrings_(pattern, first, second) {
    asserts.assert(pattern, 'List pattern must be defined');
    return pattern.replace('{0}', first).replace('{1}', second);
  }
Replaces the {0} and {1} placeholders in a pattern with the first and
the second parameter respectively, and returns the result.
It is a helper function for goog.i18n.listFormat.format.

@param {string|undefined} pattern used for formatting.
@param {string} first object to add to list.
@param {string} second object to add to list.
@return {string} The formatted list string.
@private