* Initial commit of the Dart SDK autoroller for Flutter.
The autoroller complements the existing functionality of dart_roll_helper.py by
automating the SDK revision selection process, git, and GitHub steps of the roll
process.
* Fixed bugs; Added get_dart_roll_revisions.py tool
* Fixed bugs, updated commit / PR description messages, don't copy Skia
licenses as part of roll.
* Update copyright date
