It already supports WebAssembly. All browsers do.

Here's a demo of Dart and Flutter compiled to WebAssembly:

https://flutterweb-wasm.web.app/

WebAssembly enables you to use any language. And when you can use any language, why would you use JavaScript?

Google has started migrating parts of Google Sheets to WebAssembly. They're compiling Java to WebAssembly and seeing a 100% performance increase:

https://web.dev/case-studies/google-sheets-wasmgc

Amazon has been migrating its Prime Video app from JavaScript to WebAssembly. They're compiling Rust to WebAssembly and they've seen increased performance and lower memory usage:

https://www.amazon.science/blog/how-prime-video-updates-its-...