unnecessary_breaks fails to report an unnecessary `break` inside a `default` clause

[stereotype441]

Describe the issue
Currently, the unnecessary_breaks lint only reports for break statements that appear after a case clause. If the break statement appears at the end of a default clause, no lint is reported.

To Reproduce

main(List<String> args) {
  switch (args.length) {
    case 1:
      print('1 arg');
    default:
      print('${args.length} args');
      break; // No lint reported
  }
}

Expected behavior
A lint should be reported at the line marked No lint reported above.

Additional context
Note that in the above example, if default: is changed to case _:, the lint works properly.

[stereotype441]

I'm going to look into this because it's complicating my work on #54575.

[stereotype441]

Fixed by 045d26b.