Add call to flush() to avoid flakiness

tests/standalone/io/process_stdin_transform_unsubscribe_test.dart

Index: tests/standalone/io/process_stdin_transform_unsubscribe_test.dart
diff --git a/tests/standalone/io/process_stdin_transform_unsubscribe_test.dart b/tests/standalone/io/process_stdin_transform_unsubscribe_test.dart
index 8b10d734b99d73a796ec33878c5fed51831b3bd9..2730ae0eaa9e096f9298058b71c1c6b6fc1771e1 100644
--- a/tests/standalone/io/process_stdin_transform_unsubscribe_test.dart
+++ b/tests/standalone/io/process_stdin_transform_unsubscribe_test.dart
@@ -25,6 +25,9 @@ void test(Future<Process> future, int expectedExitCode) {
     process.stdout.listen((_) {});
     process.stderr.listen((_) {});
     process.stdin.writeln("Line1");
+    process.stdin.flush().then((_) {
+      print("flush completed");
+    });
   });
 }