Add an option for starting a detached process with stdio connected

tests/standalone/io/process_detached_script.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.
 //
 // Simple script hanging for testing a detached process.
 
+import 'dart:io';
 import 'dart:isolate';
 
-main() {
+void main(List<String> args) {
   new ReceivePort().listen(print);
+
+  // If an argument 'echo' is passed echo stdin to stdout and stderr.
+  if (args.length == 1 && args[0] == 'echo') {
+    stdin.fold([], (p, e) => p..addAll(e)).then((message) {
+      stdout.add(message);
+      stderr.add(message);
+      stdout.close();
+      stderr.close();
+    });
+  }
 }