fix: check a constructor exists before instantiation

[dsgallups]

Attempts to fix #13015

Summary of issue:
When using vite-plugin-wasm with vite-plugin-top-level-await, an external async IIFE is passed alongside the generated component constructor output. Until this top level await function completes, the constructor is undefined. Svelte 5's compiler changes assume, for root.svelte, that if constructor[1] is not undefined, constructor[0] is not undefined. This is true in almost all cases, excepting where vite-plugin-top-level-await is used.

It seems like Svelte 4's compiler had a fallback constructor when the passed constructor was undefined. Please let me know if the solution is better situated within the Svelte 5 compiler, or perhaps belongs elsewhere! Thanks for your consideration!

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  • I would be happy to create an e2e test for this using vite-plugin-top-level-await in /tests/apps

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🦋 Changeset detected

Latest commit: 475f4c0

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Patch

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13025-svelte.vercel.app/

this is an automated message

[benmccann]

Hmm. This feels a bit like changing SvelteKit to fix an issue in a Vite plugin. I'm pretty uncomfortable with this. I'll take the discussion to the issue since there are more details there

[dsgallups]

Thought so too, seems like a pretty invasive change. Thanks for your advice in the issue!