[native_assets_cli] `build.dart` / `link.dart` web backends and `WasmCode` and `JsCode` assets

[dcharkes]

At some point, we'd like to output assets for web:

• DataAsset (https://github.com/dart-lang/native/issues/154)
• WasmCode (https://github.com/dart-lang/native/issues/156)
• JSCode

And WasmCode and it should have a dialect field that is linear_memory, wasm_gc. (I'm assuming the linking might work differently for the two dialects.)

Besides asset types to output, we should also have the BuildConfig communicate that we're targeting "web" instead of "native". The question is how we should surface this in the protocol and Dart API. And this depends on differences in target.

I believe build.dart and link.dart should consider dart compile js the same as flutter build web and dart compile wasm the same as flutter build web --wasm. I believe it would be an anti-pattern if build.dart or link.dart would branch on Dart vs Flutter, if we want packages to work both in Dart and Flutter. (It's fine to branch on web vs Android vs MacOS.) https://github.com/dart-lang/native/pull/946#discussion_r1516558030

@mosuem suggested to extend the API with: OS.web, Architecture.js and Architecture.wasm (and extend the protocol ditto).

I don't know enough about dart2js/dart2wasm and Flutter for web to know if it's reasonable to let users branch on whether we're compiling dart to js or wasm. I'm thinking that theoretically it might be enough to just introduce OS.web, and have the dart-js-interop and dart-wasm-interop work for any backend (js or wasm). However, that would require the dart code interoperating with js and wasm to work with both the js and wasm backends. In practice, I expect dart2js to not support wasm interop but to require a wrapper JS layer. Therefore the build.dart would need to output that wrapper when targeting Architecture.js. Hence, we need to introduce Architecture.js and Architecture.wasm alongside OS.web.

@eyebrowsoffire @mkustermann @osa1 Could you comment on how WASM and WASM GC code files are linked into a flutter build web --wasm / flutter build web / dart compile wasm application? (I'm aware that we can currently only have a single linear memory module, but conceptually how would this work?) Pointers to examples and the implementation in the Dart and Flutter SDK would be greatly appreciated. 🙏

@sigmundch Who is the right person to ask about the (new) JS interop? Does the (new) JS interop work seamlessly when compiling Dart to JS and compiling Dart to WASM? Does dart compile js have any support for including WASM code in a web app? (e.g. if Architecture.js should we allow outputting WasmCode assets at all?

Note, we don't need to answer all these questions now, or commit to supporting this. I just want to make sure that while designing the API we design it in a way so that we can support these use cases later.