Route showNotification() URLs through a fetch event?

[alastaircoote]

Right now (on Firefox and Chrome, at least) any image, icon and badge URL passed into showNotification() goes straight to the remote URL. It would be a much better experience if those requests were routed through the fetch event, so that we could use cached assets instead.

There is a work-around using cache.match() -> res.blob() -> FileReader.readAsDataURL(), but it's pretty inefficient since the lack of URL.createObjectURL (as removed in #688) means that every image is turned into a base-64 encoded string. Since the introduction of the image attribute, some of these images can be pretty huge.

[jakearchibald]

We've already made this change in the spec, we're just waiting for browsers to catch up.

[alastaircoote]

Fantastic news. Thanks!