I would have thought it would be more feasible to create something with already proven technology...is there something in Dart that I am missing which makes it considerably better than languages such as JavaScript etc.?
'Incredible immature' is plain wrong. Dart is quite mature.
The biggest disadvantage is the availability of libraries (packages) but otherwise it is very comfortable to work with. There are already packages for most requirements but the available selection is still small and they often don't have all the features you are looking for.
To your question: A few points are listed here http://stackoverflow.com/questions/7712107 or https://www.dartlang.org/support/faq.html
If everyone felt that way, nothing would ever get proven and we'd still all be using, I don't know, COBOL or something. :)
Different teams have different perspectives on risk. Some want to ensure they can ship something stable with no surprises. Others think there's a chance that a newer piece of technology will be better enough to be a competitive advantage and are willing to take that chance.
The first soldier to run onto a battlefield with a musket was using unproven technology too, but it worked out pretty well when the enemy just had swords.
> is there something in Dart that I am missing which makes it considerably better than languages such as JavaScript etc.?
I've worked on large JavaScript codebases, and I found them really hard to maintain and understand. I'm on the Dart team, so I'm obviously biased, but here's what I like:
1. The language is statically shaped, meaning classes and methods have a declarative, not imperative syntax. It means that tools can understand the classes and operations in your program just by parsing it. That makes nice code navigation (go to definition, find callsites, etc.) easy where I've never found it to work well in JS.
Note that this isn't about static types and type annotations and stuff like that. It's just about being declarative versus imperative. I also find declarative code much easier to read than JS's soup of SomeType.prototype.someMethod = function() { ... }
2. It has a really nice standard library. Solid collections with lists, maps, sets. Lots of higher-order functions for transforming them. Futures and asynchronous streams are built right into the core library (and used by the DOM!). A much cleaned up DOM library. For basic mucking around with data, I don't need to lean on third-party libraries in Dart like I do in JS.
3. Static typing help find a lot of my bugs early. I don't annotate local variables, but even annotating at method boundaries is enough to find all of my dumb typos. It also gives me auto-complete for methods, which is a huge time-saver when working with unfamiliar code.
4. Lots and lots of little nice language features: getters and setters, final, lazy-initialized top level variables, string interpolation, => lambdas, correct binding for this, solid block lexical scope, etc.
I would ask one simple question back -- Have you try Dart yet? I bet you don't. Like most people, I used to argue if Dart is too young to be production, but after I spent couple hours during weekend, all doubts is gone. And what is even magical is that I don't need to Learn it, very soon I found myself already making things I want with Dart, in a very productive way.
Dart is significantly better than JavaScript (IMO, of course), that's what it was designed for. Mostly it comes down to sane semantics and static analyzability.