[gardening] Ensure standalone/io/non_utf8_output_test flushes stdout

This can remove flakiness of the test, the approval status will be NonUtf8Output.

Change-Id: Ie911547481ed0421632a54a3a74ce93984656948
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/141123
Reviewed-by: Teagan Strickland <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: mkustermann

tests/standalone/io/non_utf8_output_test.dart

@@ -7,7 +7,8 @@
 
 import 'dart:io';
 
-main() {
+main() async {
   String german = "German characters: aäbcdefghijklmnoöpqrsßtuüvwxyz";
   stdout.add(german.runes.toList());
+  await stdout.close();
 }

tests/standalone_2/io/non_utf8_output_test.dart

@@ -7,7 +7,8 @@
 
 import 'dart:io';
 
-main() {
+main() async {
   String german = "German characters: aäbcdefghijklmnoöpqrsßtuüvwxyz";
   stdout.add(german.runes.toList());
+  await stdout.close();
 }