According to the history of the AUTHORS file (commits are not properly attributed in the repository, as far as I can see) there were 12 "external contributors" in the first year of dart (as a public project anyway): https://code.google.com/p/dart/source/browse/branches/bleedi...
Good question, I'd have to run some queries. dart.googlecode.com and github.com/dart-lang are the two main locations. We receive more patches for the libraries and docs than we do for the VM. But we have received patches for dart2js (a fairly complicated bit of code).