Dart is usable now. ES6 has long been delayed, and who knows when ES7 will be available for mass usage.
There are lots of intro pages out there to get you a little more familiar with it [0][1][2]. Dart as a language spec isn't all that deep, and it tries to keep things simple and easy to use. A lot of the power I feel like comes from the library and tools that come with the language.
To add a few more useful links. Language spec[0][1] is available under a free license and they welcome input on it[2]. The 3 top starred bugs in their bugtracker are: enums, non-nullable types, and "await" support [3], though I don't expect non-nullable types to ever be added, but they like hearing what is important to programmers. If there is really a killer language feature that people want, starring it will increase it's chances of being considered.
I don't really care about ES6 or ES7. The point of my comment wasn't to compare anything. I (subjectively) liked working with Dart. I did like the "batteries included" approach of Dart vs JS, but that's not really a language think so much as it is an ecosystem/leadership difference.
The designers got to mostly start from scratch (some things are there for JS compatibility) not only at the language level, but importantly also at the VM level. It's already ~ 2X faster than V8 and they believe there is room for more performance gains. Then add in all of the nice language features beyond classes and types and you have something worthwhile.
