Build should fail if builder requests non-linked assets but `hook/build.dart` emits assets with linker

[mkustermann]

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[dcharkes]

I thought we had an issue to track that we should add a validation step, but I can't find it.

There's a bunch of other things to verify:

• assetsForLinking should be empty if linking is not supported
• only output asset types that are supported by the embedder
• files mentioned in assets actually exist
• native code assets for bundling should have a supported linking type
• build hooks should only output assets in their own package namespace