Sweet Jesus, another one?
First, if you let web-designers loose with a programming language you're going to get dogshit. That's a given in any language, as anyone whose done contract Perl work will attest to. JavaScript is a very capable language and you can design complex object hierarchies with it if you know what you're doing, complete with encapsulation, inheritance and the lot.
Second, JavaScript is actually ECMAScript, a widely implemented (Apple, Microsoft, Google, Mozilla, Opera & others) international standard. DART is a proprietary solution pushed by one vendor. Really, does this remind you of anyone? ANYONE?
WTF is up with Google lately - Dart, WebP and (to a lesser extent) SPDY are all Chocolate Factory inventions to "improve" the web, but they're all single-vendor solutions designed to usurp existing multi-vendor ones. If successful they'll drag the web back to the bad old days.
(Incidentally the fact they're open source doesn't matter a damn. I could open source my own image format tomorrow, but that doesn't mean I should expect Mozilla and MS to implement it).
The more powerful a company becomes, the more it ignores the very standards that helped it grow. Same as it ever was.
/rant