Google released a new version of the Dart2js compiler, whose generated JavaScript which they claim outperforms a “hand-written JavaScript” in the DeltaBlue benchmark. Skipping for a moment the difficulty of defining “hand written” Dart is still dragging it's heels in other benchmarks for the Richards benchmark, the other benchmark that the Dart site publishes, the Dart-generated JavaScript performance for the Richards benchmark is still 26% slower than hand-written JavaScript.

Perhaps more interestingly is to compare Dart with other languages in which it is doing pretty well

http://www.dartlang.org/performance/