use older 2.0.4 version of whatwg-fetch in browser core package

[neonguru]

This resolves issue #186
The whatwg-fetch module is responsible for the window is not defined error.
By using the previous version of that module, the error does not occur and everything works as required.
Tested in my SSR application and it is working as intended.

[adamchel]

Haven't had the time to look at this too closely yet, but @neonguru do you have any idea what changed in whatwg-fetch between 2.0.4 and 3.0.0 that caused the window is not defined error? Would it perhaps be better to open an issue/PR with whatwg-fetch itself?

[adamchel]

I'm hesitant to downgrade a dependency without understanding what changed.

[neonguru]

There is an issue in their repo already regarding this:
JakeChampion/fetch#657

[neonguru]

But it's going on a year that they've known about it...