My Dart thingy is commercial NDA bla bla bla - so the only thing I can say it's more than 20K LOC. I was always afraid that such a big app couldn't be mantained in lang without proper IDE on non-windows (for Dart I'm using Dart's team Eclipse plugin and/or Webstorm plugin).
In my case I'm just too lazy to code in lang without lexical this and as I understand JS is TS subset, so you still can shot yourself in the foot really easy, also from what I hear JS/TS is better suited for small projects whereas Dart shines when project gets bigger (tree shaking, proper SDK) and perf really matters. On the con side: I would love Dart had as good IDE/plugin as TS has on Windows (Visual Studio).