Anything statically typed can easily beat js at speed,
there is no magic here. Obvious problem with dart is that on almost every browser it will be translated to js.
Also you want to compare it with asm.js.
I was think more about Server-side speed(Dart vs NodeJS in server-side performance). Dart has already proven that its faster than hand-written JS with dart2js in client-side.
Dart's not statically typed. The type annotations are not used at all in production mode. In checked mode they're only used for run-time type checks.