I think this is the WACK tool being dumb; data.js actually contains JSON data, but it tries to validate it as Javascript code and fails. I don't know why, I thought JSON was a strict subset of JS code...
We can't just rename data.js to data.json because a lot of servers don't have a MIME type set for .json, so return 404 not found, and so will fail to run the game.
For the next beta I'll have a workaround and only use the .json extension on fussy platforms, but I think it would be worth telling Microsoft about this as well.