ServiceWorkerGlobalScope gets fetch() from WorkerGlobalScope implementing WindowOrWorkerGlobalScope:

https://fetch.spec.whatwg.org/#fetch-method
https://html.spec.whatwg.org/#windoworworkerglobalscope-mixin

But WorkerGlobalScope doesn't have fetch, even though WindowOrWorkerGlobalScope does. It doesn't seem like there's a link between the two of them. Should WorkerGlobalScope inherits from WindowOrWorkerGlobalScope?

Also, we need to include the spec links to fetch in ServiceWorkerGlobalScope.idl

Never mind, I didn't see 
Window implements WindowOrWorkerGlobalScope;
WorkerGlobalScope implements WindowOrWorkerGlobalScope;
in the spec.

But shouldn't we be moving fetch from ServiceWorkerGlobalScope to WorkerGlobalScope or even higher?

WorkerFetch (implemented as GlobalFetch) defines fetch on partial interface WorkerGlobalScope (and the ServiceWorkerGlobalScope fetch override just calls the GlobalFetch version anyway, so removing that should have no effect afaict).

I have confirmed it is defined in Blink here:

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/serviceworkers/ServiceWorkerGlobalScope.idl

But is the difference actually visible to user code?

These two partial interfaces together should be identical to what the spec says, and could probably be refactored into a "partial interface WindowOrWorkerGlobalScope" if one was so inclined:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/fetch/WindowFetch.idl
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/fetch/WorkerFetch.idl

The override in ServiceWorkerGlobalScope is in addition to what's required to exist in the spec, and may be observable to scripts. The ServiceWorkerGlobalScope interface looks like it's exposed, and maybe ServiceWorkerGlobalScope.prototype.hasOwnProperty('fetch') returns true when it shouldn't?

I'm interested in this issue.

Work items:
1) Add WindowOrWorkerGlobalScope.idl into core/frames, and move the common attrs/functions between Window.idl and WorkerGlobalScope.idl into WindowOrWorkerGlobalScope.idl (e.g., 'origin' attr)
2) Implement "partial interface WindowOrWorkerGlobalScope" under modules/fetch and remove WindowFetch.idl and WorkerFetch.idl.
3) Remove fetch() from ServiceWorkerGlobalScope.idl.

WindowOrWorkerGlobalScope.idl will be added by https://codereview.chromium.org/2798433003/

I meant to forward this issue to a 20% contributor but she is now busy with other issues, so let me return this to the shelf for now...