They are pretty big. At work I work on a Django-backed project that has a very fat JavaScript front-end client, and JavaScript is incredibly painful to work with compared to Python.
It's not merely aesthetics, but JavaScript in the browser is severely lacking in language features that allow it to scale to larger code-bases.
Also JavaScript has a pretty good variety of languages that compile to it. If you like C#/Scala/Swift use TypeScript. If you like Java use Dart. If you like Lisp ClojureScript.
Dart comes with a batteries included standard library inspired by Smalltalk (https://www.dartlang.org/docs/dart-up-and-running/ch03.html).
Dart should also feel familiar to people coming from C#. async/await support in Dart was designed by Erik Meijer (who worked on the feature for C#) and iirc he believes it is the Dart implementation is the cleanest implementation yet.
I don't see much similarity between Dart and Java.
Since that I have had to do a fair bit of python, php and JavaScript at work. I think if I hadn't seen Dart first I would have quite liked python.
The problem with Dart is that once you've been developing in it for a while it spoils you from liking anything else. I really cringe with python, JS etc autocompleting to a million irrelevant things.
And going back to Java you have the slow dev loop that comes with a compiled language.
At the end of the day I've simply never had a development experience anywhere near as productive and enjoyable as Dart.
However, it turns out having an AST of your Javascript code is really useful, and lets you do things like this: https://github.com/speedskater/babel-plugin-rewire
Give it a try. I like that it comes with a useful and well designed standard library (like Python).
It was a dream to use compared to JS. One show-stopper though: browser support. At the time, IE11 was just released and Dart had issues with IE11. Also, the Dart supported browser window is only the latest 2 versions of a given browser.
This totally killed our Dart prospects, as our target users in our business are generally non-tech savvy and we need to support older browsers like IE8/IE9.