Offline page not working

The offline page is currently not working for amp.dev. Here is our [serviceworker](https://amp.dev/serviceworker.js):

```
importScripts('https://cdn.ampproject.org/sw/amp-sw.js');
AMP_SW.init({
assetCachingOptions: [{
regexp: /\.(png|jpg|woff2|woff|css|js)/,
cachingStrategy: 'CACHE_FIRST',
}],
offlinePageOptions: {
url: '/offline.html',
assets: [],
},
});
```

Steps to reproduce:

1. visit amp.dev
1. open DevTools and check if sw is installed and `/offline.html` is cached
1. Trigger offline mode in DevTools
1. Reload the page => works
1. click a link to an uncached page
=> no offline page is shown