Thanks for thinking about helping with Stagehand! Here are a couple of ways that you can contribute:
To avoid wasting your time, please talk with us before you make any nontrivial pull request. The issue tracker is a good way to track your progress publicly, but we can also communicate other ways such as email and Gitter.
If you make changes to template files, recompress the template:
pub run build_runner build
Travis runs these too, but your PR will look better if all tests pass the first time:
pub run test dart test/validate_templates.dart
For more information on contributing to Dart, see the dart-lang/sdk Contributing page.