Wasm/web targets

[sneurlax]

Hi @knopp, thank you for native_toolchain_rust and cargokit, I use both and appreciate your work very much.

Now I am looking at supporting web/browsers via Wasm in addition to desktop and mobile platforms. There are some issues blocking that progress:

• [ffi/js-interop/wasm] Unified API for using dart:ffi and JS Interop with WASM dart-lang/sdk#46690
• Provide interop between Dart and WASM dart-lang/sdk#37355
• [native_assets_cli] build.dart / link.dart web backends and WasmCode and JsCode assets dart-lang/native#988
• Compilation of native code to WASM dart-lang/native#156

but I think I'll be able to get a manually-built Rust crate working with Dart on the web using https://github.com/cloudpeers/ffi-gen or similar. wasm_ffi provides an example for an alternative to dart:ffi until ffi merges web support, and on the Rust side, targeting wasm32-unknown-emscripten may get us most of the way there.