I've seen projects use a compiler similar to this, and while writing C# sounds great to developers who already know C#, the differences between the C# source and the generated JS make debugging and maintenance difficult.
In my experience typescript provides C# developers much of what they are looking for - type safety, familiar class definitions, etc - while still being very easy to debug.
My "Show HN" submission yesterday[0], the web app for helping homeless youth find beds and shelter[1], is written in TypeScript.
[0]: https://news.ycombinator.com/item?id=8886588
[1]: http://ysnmn.org
Like many others I think the future of JavaScript is going to be treating it like bytecode. Write your application in whatever language you want for developer convenience and debugging, them push it out to javascript for processing. Seems to be where things are headed.
But if you're looking to write C# in the browser because you love C# and have heard JS is terrible (a BIG factor I've seen used when pushing tools like this), I'd look at typescript first.
Isn't this solved with source maps?
> DuoCode automatically generates source mappings
Also TypeScript is supported by IntelliJ family of IDEs.