function polling (opts) {
	  var xhr;
	  var xd = false;
	  var xs = false;
	  var jsonp = false !== opts.jsonp;
	
	  if (typeof location !== 'undefined') {
	    var isSSL = 'https:' === location.protocol;
	    var port = location.port;
	
	    // some user agents have empty `location.port`
	    if (!port) {
	      port = isSSL ? 443 : 80;
	    }
	
	    xd = opts.hostname !== location.hostname || port !== opts.port;
	    xs = opts.secure !== isSSL;
	  }
	
	  opts.xdomain = xd;
	  opts.xscheme = xs;
	  xhr = new XMLHttpRequest(opts);
	
	  if ('open' in xhr && !opts.forceJSONP) {
	    return new XHR(opts);
	  } else {
	    if (!jsonp) throw new Error('JSONP disabled');
	    return new JSONP(opts);
	  }
	}
Polling transport polymorphic constructor.
Decides on xhr vs jsonp based on feature detection.

@api private