Screenshot Tests

We need tests!

Testing this is complicated because of the service worker. Ideally we'd test the service worker under a number of the conditions that cause it to reload: initial visit, second visit, repeat visit with a stale worker, interacting with a page after the worker has been shut down, etc. The Workbox team might have some resources to help drive these scenarios.

We also need basic interaction tests: edit a file, see the change. And we need some visual regression tests.

We should probably look at using Web Test Runner, and use whatever it recommends for visual snapshot testing, if it does.