[element model] migrate tool to opt-out semantics

See: https://dart-review.googlesource.com/c/sdk/+/393482

Change-Id: I08fc376892652699d1eed674d8267851e2c3a02b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/393483
Reviewed-by: Konstantin Shcheglov <[email protected]>
Auto-Submit: Phil Quitslund <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: pq

pkg/linter/tool/migrate.dart

@@ -42,7 +42,7 @@ main(List<String> args) async {
     print("Writing to 'analyzer_use_new_elements.txt'...");
     print('-' * 20);
     File('analyzer_use_new_elements.txt')
-        .writeAsStringSync('${migratedFilesSorted.join('\n')}\n');
+        .writeAsStringSync('${unmigratedFilesSorted.join('\n')}\n');
   } else {
     print('Migrated files:\n');
     print(migratedFilesSorted.join('\n'));