Svelte 3, async onMount or a valid alternative?

What I need is to use async-await in Svelte onMount().

Or maybe you can suggest me what is wrong and what I can use alternatively.

To Reproduce

1. go here: https://svelte.dev/repl/000ae69c0fe14d9483678d4ace874726?version=3.23.0
2. open the console
3. click on the button
4. you should see messages: "Mounting..." and "A lot of background work..."
5. if you click again the destroy message is not written

WHY?

Did onMount() recognizes the async function promise? Should it?

I need that async behavior because I need to wait for function lazyLoading() before rendering the Child component.

Is there an alternative way to do this in Svelte?