I did use CoffeeScript after that, a beautiful little language, but only suitable for smallish projects. I need static typing for large projects.
After that, I learned Dart, and coming from C/C++, Java, AS3, it was a breeze. I find there are no quirks, everything behaves as you'd expect. Other team members were productive in Dart in about a week, without having seen a line of Dart code.
I mostly write HTML5 games and server-side with Dart, so I can't really talk too much about any client-side frameworks.
As for Dart cons, I guess one would be you have to use a different browser for web dev, but it's not really a big deal, I'm kinda reaching here. There aren't as many 3rd party libraries as some other languages. Depending on what you want, you might have to get your hands dirty and issue some pull requests for features/bug fixes for those libs. JS interop can still be a bit tricky too, depending on what you're doing.
Any specific questions, I'd be happy to answer. Also, there's a great slack channel for Dart: https://dartlang-slack.herokuapp.com/
I haven't touched Dart for over a year, but the Flutter page seems like it is on HTML, not native, and it only specifies Linux or OS X dev machine.
https://docs.google.com/presentation/d/1cw7A4HbvM_Abv320rVgP...
I'm going to try it on my Linux box. The only disadvantage I can see for my eventual needs after a second reading is the lack of 3D support. I can get by with 2D for most of my stuff, but for games, I need the 3D hook.
I'll have to see how easy it is to customize the Dart-provided widgets. Might be easier than creating them from scratch in OpenGL via the NDK.
What about using a combination of StageXL and ThreeJS? Then use Cordova to package your HTML5-based app?
http://www.stagexl.org/ http://threejs.org/ https://cordova.apache.org/