Not very different to having native support for e.g. CoffeScript in browser X and supporting other browsers via JS.
Of course the main issue is that this is yet another language. Since all browsers are essentially becoming poor man’s virtual machines, why not drop the pretense and adapt some kind of bytecode instead. There’s JVM/CLR/LLVM/NaCL — just pick one already! Alas, politics (Oracle/Microsoft/Apple/Google).
Well anyway, Dash is a very positive news nevertheless. The biggest issue with current cross-compilers (GWT, Pyjamas, Haxe etc.) is that they lack good tools to develop and debug. Having support for those natively, there’s hardly any reason to target JS directly anymore. And once Chromium has the infrastructure to support more than one language, it’s possible that the abstractions will be general enough to add support for other languages as well.
“Our approach is to make an absolutely fantastic VM/Language and development environment and build great apps that fully leverage it in order to help other browsers see the wisdom in following. Once Dash has had a chance to prove its stability and feasibility, we are committed to making Dash an open standard with involvement from the broader web community.”