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

tests/standalone/io/process_detached_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // OtherResources=process_detached_script.dart
 
 // Process test program to test detached processes.
 
 import 'dart:async';
 import 'dart:io';
@@ skipping 29 matching lines @@
   var future = Process.start(
       Platform.executable,
       [script, 'echo'],
       mode: ProcessStartMode.DETACHED_WITH_STDIO);
   future.then((process) {
     Expect.isNotNull(process.pid);
     Expect.isTrue(process.pid is int);
     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])
         .then((values) {
           Expect.listEquals(values[0], message);
           Expect.listEquals(values[1], message);
         })
         .whenComplete(() {
           Expect.isTrue(process.kill());
         });
@@ skipping 16 matching lines @@
       asyncEnd();
     });
   });
 }
 
 main() {
   test();
   testWithStdio();
   testFailure();
 }