I don't know if it is official or not, but there is a SharePoint CDN where you can get the common scripts for the JSOM, and probably (not verified) a lot of usefull resources like icons, and all the "_layouts" files.

You can reference the scripts with this URL:

https://cdn.sharepointonline.com/9109/_layouts/15/

Where you can choose http or https, the 9109 I suspect is the patch level, and finally the _layouts...

Very usefull to créate provider hosted apps...