Google Apps use Closure, and after taking a peek at the source of Microsoft's online office apps, they appear to be using Script# or a similar C# to JS tool, as

Google Apps use Closure, and after taking a peek at the source of Microsoft's online office apps, they appear to be using Script# or a similar C# to JS tool, as I see lots of references to namespaces, and lots of calls to dispose() methods. iCloud's office apps use Sproutcore, which eventually forked into Ember (though the original Sproutcore project still exists).