Oddly ms already has/had some sort of 'compiler'. Memory is fuzzy on it, internal use and only used to build one thing, libs for messenger in browser I think.
You're thinking of Script# (http://scriptsharp.com/) and it's used at the very least to develop the Office web apps as well as Exchange's Outlook Web Access.
I'd suspect they have several tools like this for code portability. It's relatively trivial to make a simple translator that compiles to Javascript; take CoffeeScript for example. I'd be surprised if there weren't all sorts of tools like these floating around Redmond. The notion of a "runtime" sitting on top of Javascript? That doesn't even make sense. It would also be a waste of Anders's strengths. His strength is in taking something popular and making it modern and usable (e.g. Java -> C#). Coming from him, I'd expect more along the lines of a framework or language extension or ECMA contribution than a runtime or compiler.
IIRC they employ the guy who wrote Script# and they use it for some projects.