Don't close stdio/stderr when shutting down.

tests/standalone/io/stdio_implicit_close_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:io";
+
+void main(List<String> arguments) {
+  // Access stdout and stderr so that the system grabs a handle to it. This
+  // initializes some internal structures.
+  if (stdout.hashCode == -1234 && stderr.hashCode == (stderr.hashCode + 1)) {
+    throw "we have other problems.";
+  }
+
+  if (arguments.contains("stdout")) {
+    stdout.close();
+  }
+  if (arguments.contains("stderr")) {
+    stderr.close();
+  }
+}