I think google missed big time on Dart, and something like TypeScript is what they should have done. I really find it hard to come up with bad things to say about Microsoft's move, it's a mental cognitive dissonance, but I have to admit, they are doing something very right.

1) it's just JavaScript - winning some CoffeeScript fans

2) regular JavaScript is valid, (just like SCSS vs SASS)

3) it has day 1 interop with vanila JS (Dart just started, and until it has it, it's just a play-with language)

4) TypeScript support for most popular editors (Dart has mostly Eclipse I think, TypeScript has support for everything but eclipse, but an eclipse plugin is just a matter of time probably)

5) TypeScript has a bit more chance becoming a standard, (future ECMASCript 6?)

Well played Microsoft, didn't see it coming...

VS 2012 is actually nice, still I would probably wait for an Eclipse plugin (I'm sure someone out there is already working on it) but this is for me a big reason not to start learning Dart