DAS needs impl or maybe just tests for RemoveLeadingUnderscore with variable declaration patterns

[srawlins]

dart-archive/linter#4133 added linter support for no_leading_underscores_for_local_identifiers in variable declaration patterns. DAS should have support in RemoveLeadingUnderscores as well.

[pq]

Good catch.

Confirmed:

f() {
  final (_a, b) = (1, 2);
  print('$_a$b');
}

[pq]

https://dart-review.googlesource.com/c/sdk/+/291361