Fix linter issues on dev in update_tests.dart

[athomas]

No description provided.

[athomas]

@munificent dart format still breaks (not sure what to do about this, fixing this would probably break on 2.19):

--- a/lib/src/testing/test_file.dart
+++ b/lib/src/testing/test_file.dart
@@ -14,8 +14,8 @@ final _unicodeEscapePattern = RegExp('[\x0a\x0c\x0d]');
 Future<String> findTestDirectory() async {
   var libraryUri = await Isolate.resolvePackageUri(
       Uri.parse('package:dart_style/src/testing/test_file.dart'));
-  return p.normalize(
-      p.join(p.dirname(libraryUri!.toFilePath()), '../../../test'));
+  return p
+      .normalize(p.join(p.dirname(libraryUri!.toFilePath()), '../../../test'));
 }

[munificent]

Thanks!