function buildUrl_(url, params) {
  var paramString = Object.keys(params).map(function(key) {
    return encodeURIComponent(key) + '=' + encodeURIComponent(params[key]);
  }).join('&');
  return url + (url.indexOf('?') >= 0 ? '&' : '?') + paramString;
}
Builds a complete URL from a base URL and a map of URL parameters.
@param {string} url The base URL.
@param {Object.<string, string>} params The URL parameters and values.
@return {string} The complete URL.
@private