I took a look at it when it was relatively new, I liked the possibility of using types and classes and the overall syntax.
Unfortunately its not native implemented in other browsers than chrome/chromium (which has iirc dartium).
So you need to compile it to javascript to run in other browsers than chrome.
That makes it more an abstraction layer on top of javascript helping developers used to languages like C# or Java
that dont want to learn javascript since it has another structure than the aforementioned.
That was also the reason i stopped using it, since when I need to compile javascript out of it i can use javascript directly and use its unique traits and features
without having dart in the way.
Just my personal opinion on it.