I haven't, but if it has most of the things I mentioned as things I liked, it'd likely be a capable substitution (for me). Dart is good enough for me, though. I really like their approach as a whole.

To be honest, if XBox Music needs to be implemented in 100k lines of a very high level language there's something very wrong going on.

You clearly don't know how complicated a music player + music store + music library manager can get. There's a bunch of stuff going on there between music codecs, library management, streaming music downloads, pulling id3/etc tags down from db servers, copying music to mobile devices, managing playlists...

I've written game delivery systems which were smaller, like 1/20th of the size.

You aren't exactly going to be writing the CODECs and DRM for XBox Music in TypeScript are you?

Using it successfully on large team. Works great and plays well with all the existing JS libraries.