function toASCII(domain) {
		return mapDomain(domain, function(string) {
			return regexNonASCII.test(string)
				? 'xn--' + encode(string)
				: string;
		});
	}
Converts a Unicode string representing a domain name to Punycode. Only the
non-ASCII parts of the domain name will be converted, i.e. it doesn't
matter if you call it with a domain that's already in ASCII.
@memberOf punycode
@param {String} domain The domain name to convert, as a Unicode string.
@returns {String} The Punycode representation of the given domain name.