jsFiddle how to get json?
What you are experiencing is an exception being thrown by the XMLHttpRequest object, because you are using AJAX to call elements from different domain. This is better said, for example, in here:
"The XMLHttpRequest object is prevented from calling web services from outside its own domain. This is sensible given that if you called a script in one place and it, in turn, called a script on another server, it could leave an application open to all sorts of malicious scripts, hacks and exploits."
So the easiest way to do is to code it locally and call a local copy of the files (poly.json, line.json and point.json) that would reside on your local server. For testing if all displays on a map you could hard-code the files into your code. I am not sure how it could be achieved otherwise.
A good solution can be this : using github responses . You can store your example in github, along with the predefined XHR requests responses.