A minimal reproduction of an issue with custom svelte stores

To reproduce:

npm install
npm run build

You should see output that looks like this:

{ a: Set(0) {} }
{ b: undefined }
{ c: Set(0) {} }
{ d: Set(0) {} }

The fact that b is undefined is the bug!

Now go into +page.svelte and delete lang="ts" from the script tag.

Run npm run build again and...

{ a: Set(0) {} }
{ b: Set(0) {} }
{ c: Set(0) {} }
{ d: Set(0) {} }

🤯