Getting exception when using iron-ajax for sync request.

[anotherapj]

Description

The iron-ajax element is throwing script exception for sync request.

Expected outcome

The iron-ajax should complete the sync request and return the request object with response data.

Actual outcome

Getting the below exception
InvalidAccessError: A parameter or an operation is not supported by the underlying object

Live Demo

NA

Steps to reproduce

1, Create iron-ajax element.
2, Set the sync property.
3, call the generateRequest() method.
Below is the code(slightly modified version from app-localize-behavior)
`var ajax = proto.__localizationCache.ajax;
if (!ajax) {
ajax = proto.__localizationCache.ajax = document.createElement('iron-ajax');
ajax.sync = true;
}

var request = proto.__localizationCache.requests[path];
if (!request) {
    ajax.url=path;
    ajax.method= "POST";
    ajax.headers = { "Content-Type":"application/json"};
    ajax.body = this.getRequestBody();
    ajax.handleAs = "json";        
    request = ajax.generateRequest();`

The failure is happening on iron-request.html file @ send method, trying to set
xhr.withCredentials = !!options.withCredentials;
If I remove the above line, the request works fine.

Browsers Affected

• [ y] Chrome
• [ y] Firefox
• Safari 9
• Safari 8
• Safari 7
• Edge
• IE 11
• IE 10