Fixes project to work with Dart v1.0.0.3_r30188 (and, hopefully, higher)

This project doesn't compile with Dart version 1.0.0.3_r30188 due to a breaking change that removes a method called "runAsync".  I'm not entirely sure about the details since I'm trying to learn Dart, but I managed to get it working by  upgrading the dependencies (path and unittest) to their latest versions and hacking together a fix for the change in the "path" API (namely, Builder was removed and replaced with Context).