[ServiceWorker] importScripts() should throw? (#642)

I thought the idea was that `importScripts()` was only available during the installation event as it's a synchronous API. And that outside of that specific context we'd make it throw. It seems sad to allow such a synchronous all the time.

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/642