Dart solves the wrong problem. At the end of the day, it just boils down to yet another syntactic abstraction over javascript. The real problems with javascript are that it is not really suitable as a low level programming language for web applications. Javascript source code is too large, the run time is too heavy, the memory consumption is too much and it is really hard to optimize. While it may have been a reasonable language for scripting small tweaks in web apps, it is a pretty bad platform for writing optimized performance driven applications. Something like a properly integrated and sandboxed JVM/LLVM platform is what the web really needs -- not yet another slow scripting language. It is a shame that not much seems to be happening with NACL these days, since that project is technically speaking a way better solution than Dart.
RE: "Something like a properly integrated and sandboxed JVM/LLVM platform is what the web really needs"
I agree with you 100%. Why is it such an issue to implement something like this in a browser?