Revert "Revert "Make stdout/stderr async""

tests/standalone/io/stdio_nonblocking_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.
-
-import "dart:convert";
-import "dart:io";
-
-import "package:expect/expect.dart";
-
-void main() {
-  var script =
-      Platform.script.resolve("stdio_nonblocking_script.dart").toFilePath();
-  Process.run(Platform.executable,
-              ['--checked', script],
-              stdoutEncoding: ASCII,
-              stderrEncoding: ASCII).then((result) {
-                  print(result.stdout);
-                  print(result.stderr);
-    Expect.equals(1, result.exitCode);
-    if (Platform.isWindows) {
-      Expect.equals('stdout\r\n\r\ntuodts\r\nABCDEFGHIJKLM\r\n', result.stdout);
-      Expect.equals('stderr\r\n\r\nrredts\r\nABCDEFGHIJKLM\r\n', result.stderr);
-    } else {
-      Expect.equals('stdout\n\ntuodts\nABCDEFGHIJKLM\n', result.stdout);
-      Expect.equals('stderr\n\nrredts\nABCDEFGHIJKLM\n', result.stderr);
-    }
-  });
-}