First, great job with doing the Dart port! I fancied prototyping a game using Dart (after attending the London hackathon a couple of weeks back) and needed Box2D. Always nice when your prereq's are waiting for you :-D
That said, I found the existing version errored with "UnsupportedOperationException: Cannot add to a non-extendable array" in "BroadPhase.dart". Looks like List's construction has changed and specifying capacity causes them to be fixed capacity and add() not to work! Switching to dynamic size fixed that [for now]. Next, webkitRequestAnimationFrame() no longer takes the canvas element. Tests fine on current Dartium that comes with Dart IDE on Linux 32-bit (in case there're differences).
I've cloned the repository and pushed 1 changeset. Let me know if/how to issue a pull-request (if such one does with git on Google Code!).
Looking forward to getting my prototype on now :-D