This step by step guide uses issue RegEx: add a way to get the positions of groups #42307 as a concrete example. You can also watch associated videos:
- Contributing a change to Dart SDK Core Libraries (Part 1)
- Contributing a change to Dart SDK Core Libraries (Part 2): Responding to a Review
Ask Dart team before jumping into the implementation. The best place to ask is on the issue tracker, you can also try hackers-dart
channel on Flutter Discord or one of the channels on Dart Community discord - but most SDK developers are not on either of them.
Why ask the team?