> Doesn't a 30% performance improvement (I'm just talking dart vs. JavaScript, not dart2js generated code vs JavaScript) seem like a pretty weak result given that they've created a new language?

Well keep in mind that every other language that compiles to JS is generally slower, so any improvement here is pretty impressive.

Note that this is just the performance of the Dart code compiled to JavaScript. Dart code running on the native Dart VM has very different, better perf.