Hi all,
In order to overcome this limitation you can host the file in dropbox, and in order to overcome the CORS issue you may use a proxy like CORS AnyWhere to make that file available in your sample app.
I did a test with the file that you mentioned. I uploaded to my dropbox and got the share link "https://www.dropbox.com/s/jx0vkkx11hvtwxn/TestPolygon.kml" in order to make this direct link you need to change the www for "dl" (direct link), so dropbox link will not present the preview of the file but returning the file immediately "https://dl.dropbox.com/s/jx0vkkx11hvtwxn/TestPolygon.kml", so this link can be retrieved by the proxy that adds the CORS headers so it can be displayed correctly in your test application:
https://cors-anywhere.herokuapp.com/https://dl.dropbox.com/s/jx0vkkx11hvtwxn/TestPolygon.kml
Hope this helps you to overcome this.
Best regards.