this is a confusing question; why would it be much better to e.g. compile a C program for x86 linux musl but not the C compiler?

Your analogy doesn't quite hold. The primary use case of a compilation target is to compile programs, not the compiler itself.

With Dart specifically, "dart compile wasm" already exists precisely for that purpose. Compiling the entire Dart VM (a multi-hundred-thousand-line C++ codebase) to Wasm and then running Dart inside that is a clever in-browser IDE trick, but it's heavy, indirect, and not what Wasm/WASI was designed to showcase. It also sidesteps WasmGC, which is exactly the kind of Wasm evolution that makes Dart-to-Wasm compelling.