DAS needs impl or maybe just tests for ReplaceFinalWithVar with a variable declaration pattern

[srawlins]

As of dart-archive/linter#4152, linter will report unnecessary_final on variable declaration patterns. DAS should support ReplaceFinalWithVar in variable declaration patterns.

[pq]

Good catch. Confirmed no fix here:

f() {
  final (a, b) = (1, 2);
}

[pq]

And not here:

[pq]

https://dart-review.googlesource.com/c/sdk/+/291342