[native_assets_cli] Validate tagged union syntax

[dcharkes]

Bug: #1826

Changes the constructors of tagged union super classes to be factories and to branch on the known sub classes. This fixes the syntax validation for tagged unions.

This PR also:

1. splits the ClassInfo to only support subclassing in NormalClassInfo (the enum classes don't allow subclassing),and
2. refactors the NormalClassGenerator in multiple methods for better readability, and
3. adds a -d (dump) parameter to the generator to dump the representation halfway in the pipeline as a text file.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
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pkgs/jni/lib/src/third_party/generated_bindings.dart
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[coveralls]

coverage: 86.091% (+0.1%) from 85.96%
when pulling 303a735 on syntax-tagged-union
into 763c3ce on main.

[HosseinYousefi]

Do we also want these classes like LinkMode to be sealed?

[dcharkes]

This is an interesting question. The question is whether we do want to allow generated syntaxes to extend other syntaxes.

Before, I was reusing class Asset from the base protocol syntax in the extensions syntax. But then I realized that we don't want to expose the syntax from package:hook, and import it in package:code_assets. So, instead now we're also generating a class Asset in the syntax for the extensions.

So now we kind of depend syntax classes not being extended anywhere else. So we could consider making every class final.

Sealed doesn't work because we do actually want objects of the class: E.g. Asset doesn't have to be a CodeAsset. So we're kind of expecting these tagged unions to be open tagged unions.

For LinkMode we could have a closed tagged union, but we currently don't distinguish between open and closed tagged unions.

Tracking in #2124

[dcharkes]

Thanks @HosseinYousefi ! 🙏