Add exitCode getter counterpart to the existing setter in dart:io.

tests/standalone/io/process_set_exit_code_script.dart

 // Copyright (c) 2013, 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";
 
 main() {
+  if (exitCode != 0) {
+    throw "Bad initial exit-code";
+  }
   stdout.write("standard out");
   stderr.write("standard error");
   exitCode = 25;
+  if (exitCode != 25) {
+    throw "Exit-code not set correctly";
+  }
 }