Support loading in a Service Worker

[matthewp]

I think this would work if https://github.com/ModuleLoader/es6-module-loader/blob/master/src/system.js#L64 checked for global.fetch and used that.

[guybedford]

Does XHR not work inside of service workers?

[OliverJAsh]

How would we download and execute SystemJS in a service worker at all?

[matthewp]

@guybedford I believe not, it's hitting the throw in this case. Eventually all environments will support fetch so it would be nice to make that the default anyways.

@OliverJAsh You can execute any script as a service worker, so that includes es6ml. You might want to bundle it with some other script that calls System.import if you want it to actually do something though.

[guybedford]

Ok, sure, yes it makes sense to use fetch if its supported as an API here anyway.

[guybedford]

I believe support for this was added in #448.