Why Dart? Why not a language agnostic runtime and then have Dart target that?
Then when some new (or old) language wants to run in the browser you don't have to update your browser for it.
I don't have to upgrade my CPU to run a new language.
I don't have to upgrade my OS to run a new language.
Why should I have to upgrade my browser?... its time that browsers have a nice interface that any code could hook into.
How about LLVM or something as a standard?
I think Google is already doing this with Native Client... though I think they sandbox/sanitize the generated machine code rather than the LLVM bytecode.