Re: [w3c/ServiceWorker] registration.ready promise (#770)

Here's a rough polyfill for those looking for one:

```js
function regReady(reg) {
  return new Promise((resolve, reject) => {
    if (reg.active) {
      resolve();
      return;
    }

    if (!reg.installing && !reg.waiting) {
      reject(Error('Install failed'));
      return;
    }

    (reg.installing || reg.waiting).addEventListener('statechange', function() {
      if (this.state == 'redundant') {
        reject(Error('Install failed'));
      } else if (this.state == 'activated') {
        resolve();
      }
    });
  });
}
```

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