default is redundant when switch is exhaustive

[srawlins]

analyzer Hint implementation issue for dart-lang/language#1539

Given:

enum Level {
  low,
  medium,
  high
}

String myFunction(Level level) {
  switch (level) {
    case Level.low:
      return 'low';
    case Level.medium:
      return 'medium';
    case Level.high:
      return 'high';
    default:
      return 'never be executed';  // 'switch' is exhaustive so 'default' is redundant here
  }
}

Show a warning that default is dead code. I suspect this would use our current dead_code mechanisms and/or flow analysis.

[shilangyu]

I would like to give this a try. Any pointers as to which packages this affects would be appreciated. I can probably figure out the rest.

[parlough]

Closing as a duplicate of #54575 and resolved by cda2815.

#54575 has many more linked commits that worked towards resolving this across the ecosystem.