
Closes https://github.com/flutter/flutter/issues/162041. I also converted the `.sh` and `.bat` files (largely) to Dart, mostly because I don't know enough Windows XP command prompt in order to make the change safely. They should still run the same from the CI bootstrapping, but most of the logic is now Dart code. Will send a separate PR to update the `flutter/tests` PR template. /cc @johnmccutchan @Piinks