> It would be nice if Google maintained mirrors on Github and accepted pull requests and such, but I don't see that ever happening.

Actually, external contributors John Evans and Ladislav Thon submitted and landed patches via github.

https://chromiumcodereview.appspot.com//10168022 https://chromiumcodereview.appspot.com//10166019

Not all of the code is on github but many libraries are. Web ui is developed via github. See here for a full list:

https://github.com/dart-lang/