Opinion: nobody should be shipping any JS to oldIE, period.
Their JS engines are woeful. Even if you can get something app-like (the kind of thing you would use dart for) to work, your app won't scale w.r.t. perf so you're painting yourself into a corner.
Some do, of course, but some of the rest of us are more interested in future advantages. If you need to target legacy browsers, legacy technologies are widely available.
If you'd like the future to be as good as possible, you might want to build new tools without the same old constraints.
I personally built a single page application about 2 years ago that worked in IE8, thanks to jQuery and the ecosystem of the time. jQuery, jQuery UI, various plugins incl. jsTree & DataTables.
It flew in Chrome, Firefox and Opera, even then. In IE8 it was slow as hell. Event listeners were slow, ajax and parsing JSON was slow. Everything was crazy slow. The app felt lumpy and awful. And when I hit bugs, the developer tools just were not there.
I would never do this again.
I'd wager that for IE < 8, for any non-trivial app, round-tripping to the server will give you a better UX than using JavaScript.
Their JS engines are woeful. Even if you can get something app-like (the kind of thing you would use dart for) to work, your app won't scale w.r.t. perf so you're painting yourself into a corner.