Adds a service worker using Workbox

[jeffposnick]

Hey @romannurik!

If you're cool with it, this PR will add a service worker that precaches all of the critical static content, and then sets up some runtime caching for the third-party Google Fonts and Analytics requests.

You can test it out at https://jeffposnick.github.io/AndroidAssetStudio/

(It doesn't add a web app manifest or handle Add to Homescreen prompts, but that could be done in a follow-up if you're open to it.)

CC: @PaulKinlan

[romannurik]

Hey Jeffy thanks for doing this!

My only concern was in @PaulKinlan's CL on another app of mine, it seemed to break browsersync reload. Would this PR have the same issue?

[jeffposnick]

I just pushed out another commit that does two things:

• Uses separate ports for dev and prod, creating two origins. That means the service worker/cache from one environment won't effect the service worker/cache from the other.

• Put in a dummy, no-op sw.js that's used in the dev environment, so even though it will be registered, it won't intercept network traffic. That should make browsersync happy.

browsersync won't work as expected in the prod environment with the real service worker, because the service worker will intercept requests and used cached responses, but that will match the real-world behavior.

[romannurik]

This is perfect thanks Jeffy!

[romannurik]

I'll see about doing another release soon that'll include this.

[jeffposnick]

Sweet! Thanks for building awesome web apps.