Add exitCode setter to set the exit code returned by the Dart VM

tests/standalone/io/process_set_exit_code_test.dart

+// Copyright (c) 2012, 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.
+//
+// Process test program to test process communication.
+
+library ProcessSetExitCodeTest;
+
+import "dart:io";
+
+main() {
+  var options = new Options();
+  var executable = options.executable;
+  var script = options.script;
+  var scriptDirectory = new Path.fromNative(script).directoryPath;
+  var exitCodeScript =
+      scriptDirectory.append('process_set_exit_code_script.dart');
+  Process.run(executable, [exitCodeScript.toNativePath()]).then((result) {
+    Expect.equals("standard out", result.stdout);
+    Expect.equals("standard error", result.stderr);
+    Expect.equals(25, result.exitCode);
+  });
+}