Very comprehensive and well-informed overview here.
From the conclusion:
<font face="Verdana, Arial, Helvetica, sans-serif"><font color="blue">"If it were still 1995, and if Netscape Inc. had to pick between JavaScript and Dart for its scripting language, Dart would plainly be the better choice. Dart has had 16 years to learn from JavaScript's mistakes; it's no wonder that it avoids most of JavaScript's pitfalls.
"But it's now 2011, and JavaScript is the language that's out there and has the support. Even if the major browser vendors committed to providing native Dart support tomorrow, it's in a sense too late; it will be decades before a browser vendor could even consider dropping JavaScript. The language is very much here to stay."</font></font>
Scirra's on the right road.