Regarding editor support, it's been great from the very beginning. A few years ago, the Dart Editor was a pleasure to use and nothing else in frontend development was even close, this was before vscode existed. It had integrated debugger and code completion worked really well, it's a bit sad that they decided to discontinue the editor, but these days WebStorm has all the features and more. There are also Atom and vscode plugins but I haven't tried them.
Debugging experience I think it's one of the highlights of Dart tooling. The observatory tool is great and it not only helps with debugging but also with profiling. I seriously recommend it.
More good tooling in Dart includes the package manager which is very well designed and easy to use, and the code formatter which just does the right thing.
Where it falls short: JavaScript interoperation. There is some, but very limited. They are working on it, but it's not quite there yet.