[w3c/ServiceWorker] activate event not working with async function (#1388)

```
self.addEventListener(
  'activate',
  function (event) {
    event.waitUntil(activateHandler())
  }
);
```

The following function does not get called, I tried with normal async but nothing is not working
```
async function activateHandler(){
  //Async code here
}
```

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https://github.com/w3c/ServiceWorker/issues/1388