LGTM, but I'd love to have a second eye on this. @windelbouwman ?

Oh, I did a quick patch in #267 to allow for testing of a WASM feature, this seems like an improved version of that.

@rmliddle did you already fix this? I believe so, of so, this PR can be closed.

Yes we can close this PR, the conditional import of the io/os modules that @coolreader18 made in #267 will still be required.

I think hiding the io/os modules (which is what @coolreader18 has done) from the wasm build completely is a much more favourable approach than trying to hide each and every method. At least until we can come up with better semantics for these modules in the browser.

Hey, I like what you all did with the os module. I updated this PR to allow compiling for --target wasm32-unknown-unknown. This PR now is a fairly uncontentious change.