If you want to also optimize the code, then binaryen can do both (wasm-opt to optimize, wasm-as to convert to binary).
Since this is for a compiler backend, then the optimizations are probably relevant for you.
However, from searching around, it looks like I'll have to emit bytecode directly, at least for the near future. Which isn't the end of the world I guess!
Yeah, I think it would have been nice to have text format input support in browsers, but the decision was against that.
But you do have other options than emitting binary wasm yourself. You can use a compiled version of wabt or binaryen, here's an example of binaryen.js,
https://github.com/WebAssembly/binaryen/blob/master/test/bin...
edit: actually it looks like the test doesn't cover WasmBinaryWriter, which is what would be needed for this, we should improve that
It uses the code from here: https://github.com/WebAssembly/wabt/tree/master/demo