when using window.fetch, even the prefetch Promise is resolved, the resource may still be loading.

**Describe the bug**
```js
prefetch(url).then(() => {
// the resource may still be loading
})
```

**Expected behavior**

```js
prefetch(url).then(() => {
// the resource have been loaded
})
```

**How to fix**

```js
export function priority(url) {
return window.fetch
? fetch(url, {credentials: `include`}).then(r => {
if (r.ok) return r.arrayBuffer()
throw new Error(r.statusText)
})
: viaXHR(url);
}
```

BTW, why include credentials ?