[Turbopack] Duplicate class definitions across route chunks, breaking `instanceof`

Link to the code that reproduces this issue

https://github.com/TrevorBurnham/nextjs-instanceof-api-error

To Reproduce

1. Clone the repo and run npm install && npm run build
2. Inspect the build output:

grep -c "class e extends d.ServiceException" .next/server/app/api/*/route.js

Output shows each route has its own class definition:

.next/server/app/api/test/route.js:1
.next/server/app/api/test2/route.js:1

Current vs. Expected behavior

Current: When multiple routes import the same class from an external package, Turbopack inlines a separate copy of the class definition into each route's bundle. This causes instanceof checks to fail across routes because each route has a different class object.

Expected: The class should be bundled once in a shared chunk and imported by all routes, preserving instanceof behavior.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.2.0
  Available memory (MB): 36864
  Available CPU cores: 12
Binaries:
  Node: 20.18.1
  npm: 10.8.2
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 15.5.10
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Turbopack

Which stage(s) are affected? (Select all that apply)

next build (local), Other (Deployed), Vercel (Deployed)

Additional context

I noticed this when working with AWS SDK clients: I had some logic that checked

if (error instanceof AccessDeniedException) { ... }

where AccessDeniedException was imported from one of those client packages. In local dev, the condition was true as expected. But in production (with the Turbopack build), it'd return false!

As a workaround, I've added all of my AWS SDK clients to serverExternalPackages for now.