No, I'm not sure. I profiled the hand-written JS function one time and noticed a lot of time spent in polymorphic methods. Also, this coincided with an optimization in GWT to improve hidden class V8 optimizations and speed went up by 300%, so it seemed related to polymorphism.

The vector math does look like an apples-to-oranges comparisons. Simple methods like cross-product will inline in GWT as well. The biggie seems to be the elision of the temporary, we'd need something like C++'s return value optimization to get rid of that.

I'll try rerunning the bench by adding calls to cross-product to see how it fares. Actually for us, we weren't as interested in comparing the speed to Dart as much as comparing it to the JVM version. The slowdown there is on the order of 50% which isn't bad. In the original thread on G+ I noted there's no way this can be considered an apples-to-apples comparison because they're not running the same code (the port's from Box2D differ), but Joel's whole Box2D benchmark suite kind of rests on this.