From this page:

Similarly, the dart2js numbers are generated by compiling the benchmarks and harness to JavaScript and then running the generated code in V8.

Finally, the V8 numbers are generated using the same benchmarks written in JavaScript and with exactly the same harness rewritten in JavaScript.

From what I'm reading, this appears to suggest that Dart2js isn't faster than V8, but is instead faster than a handwritten code. Still a good achievement!

The reason I bring this up is because I was confused at how javascript (even compiled from Dart) could be faster than the engine that is running it.