[dart:io] flush() stdin before close()ing

tests/standalone/io/process_detached_test.dart

Index: tests/standalone/io/process_detached_test.dart
diff --git a/tests/standalone/io/process_detached_test.dart b/tests/standalone/io/process_detached_test.dart
index b72f26af6328524e9bf99b9261603a162c599841..b4122526f4c5edc439f936fc382ded33aaddd73b 100644
--- a/tests/standalone/io/process_detached_test.dart
+++ b/tests/standalone/io/process_detached_test.dart
@@ -47,7 +47,7 @@ void testWithStdio() {
     Expect.isNull(process.exitCode);
     var message = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
     process.stdin.add(message);
-    process.stdin.close();
+    process.stdin.flush().then((_) => process.stdin.close());
     var f1 = process.stdout.fold([], (p, e) => p..addAll(e));
     var f2 = process.stderr.fold([], (p, e) => p..addAll(e));
     Future.wait([f1, f2])