From wingo/stringrefs#35, if we think about a browser deployment environment, we would like to avoid a situation in which two parts of a running system are implementing toUpperCase on a JavaScript string. If WebAssembly needs this or related string functionality, it should be able to use what's already in the browser.

So what's the story here? A couple options:

1. Wild west: if you know you're targetting a browser, generate an associated JS wrapper that calls the String.prototype or Intl.prototype functions that you need
2. Standard library: Regardless if you're targetting WASI or the web, you know there is a standard module you can import that provides the functionality you need. On the web, such a module delegates to String, Intl, and so on.
3. Core: There is a string.to_upper instruction. Lots more, probably.

I have to say, I see (1) as an OK short-term answer and (2) as a nice end result. I wouldn't do (3) but who knows!