Thanks for reporting this, Peter.
Fortunately, I was sitting on a pull request that had been made against the bleeding edge Dart distribution and waiting for it to hit the standard distribution. So I merged that, updated the pubspec.yaml, README and VERSION files, and tagged a new version 2.0.4 out at GitHub.
Of course, made sure the unit tests were all good, and then pushed a new version out to the Pub:http://pub.dartlang.org/packages/puremvc
So, you can remove the pubspec.lock file on your project and do another pub update, and you should be able to pull down a working version and go.
PS: Thanks Adam Singer for the pull request and being ahead of the curve on this one, I really appreciate it.