First pass at version constraint solver.

It does:
- Walk dependency graphs.
- Unify shared constraints.
- Go through Source to get dependency and version information.
- Handle the fact that everything is async.
- Find best versions that match all constraints.
- Error out if it can't find a match or gets stuck in a loop.
- Have an OK amount of tests.

It does not:
- Handle dependencies from different sources.
- Integrate with the rest of pub.
- Implement an actual "pub update" command.
- Do very much to minimize server traffic.
  (Though it does cache pubspecs it has requested.)

Review URL: https://chromiumcodereview.appspot.com//10540151

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@8985 260f80e4-7a28-3924-810f-c04153c831b5