Make http.Client work in Web workers

[kaoet]

In Web workers, there is no window object, so the current http.Client doesn't work since it explicitly calls window.fetch():

http/pkgs/http/lib/src/browser_client.dart

Lines 66 to 67 in 79470d0

	final response = await window
	.fetch(

There is WorkerGlobalScope.fetch in Web workers. Please don't hardcode window and make it also work in Web workers. Maybe like what fetch_api does:

https://github.com/Zekfad/fetch_api/blob/c3c8de685b4f35079f0c0d9dcfa259afa18e8594/lib/src/fetch.dart#L7-L8