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

pkg/testing/lib/src/stdio_process.dart

 // Copyright (c) 2016, 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.md file.
 
 library testing.stdio_process;
 
 import 'dart:async' show
     Future,
     Timer;
 
@@ skipping 36 matching lines @@
       sb.writeln();
       sb.writeln("Sending SIGTERM to process");
       process.kill();
       timer = new Timer(const Duration(seconds: 10), () {
         sb.writeln("Sending SIGKILL to process");
         process.kill(ProcessSignal.SIGKILL);
       });
     });
     if (input != null) {
       process.stdin.write(input);
+      await process.stdin.flush();
     }
     Future closeFuture = process.stdin.close();
     Future<List<String>> stdoutFuture =
         process.stdout.transform(UTF8.decoder).toList();
     Future<List<String>> stderrFuture =
         process.stderr.transform(UTF8.decoder).toList();
     int exitCode = await process.exitCode;
     timer.cancel();
     sb.writeAll(await stdoutFuture);
     sb.writeAll(await stderrFuture);
     await closeFuture;
     return new StdioProcess(exitCode, "$sb");
   }
 }