Revert "Revert "Make stdout/stderr async""

tests/standalone/io/stdio_nonblocking_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.
-
-import "dart:async";
-import "dart:io";
-
-class Message {
-  final message;
-  Message(this.message);
-  toString() => message;
-}
-
-void test(IOSink sink) {
-  sink.add([65, 66, 67]);
-  sink.write('DEF');
-  sink.writeAll(['GH', 'I']);
-  sink.writeCharCode(74);
-  sink.writeln('KLM');
-}
-
-void main(List<String> arguments) {
-  stdout.nonBlocking.writeln('stdout');
-  stderr.nonBlocking.writeln('stderr');
-  stdout.nonBlocking.writeln();
-  stderr.nonBlocking.writeln();
-  stdout.nonBlocking.writeln(new Message('tuodts'));
-  stderr.nonBlocking.writeln(new Message('rredts'));
-  test(stdout.nonBlocking);
-  test(stderr.nonBlocking);
-  Future.wait([stdout.nonBlocking.close(),
-               stderr.nonBlocking.close()])
-      .then((_) => exit(1));
-}