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.
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.