That's not how you get the source, that's how you prepare for building the whole project, including the VM, compiler, etc. If you just want the source code, just

svn checkout http://dart.googlecode.com/svn/trunk

That isn't terribly easy to find, though, it's true (you either have to have used google code before and know it's on the "checkout source" page or dig through those "get and build" instructions for just the "get" part).

The rest is just dependencies (and daunting at first, but pretty simple after you do any chromium work). You have to deal with the same thing if you check out Firefox and you want to build it (gclient here becomes bootstrap.py there).