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

runtime/bin/process_patch.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.
 
 patch class _WindowsCodePageDecoder {
   /* patch */ static String _decodeBytes(List<int> bytes)
       native "SystemEncodingToString";
 }
 
 
@@ skipping 107 matching lines @@
       native "Process_SetSignalHandler";
   /* patch */ static int _clearSignalHandler(int signal)
       native "Process_ClearSignalHandler";
 }
 
 
 patch class _ProcessUtils {
   /* patch */ static void _exit(int status) native "Process_Exit";
   /* patch */ static void _setExitCode(int status)
       native "Process_SetExitCode";
+  /* patch */ static int _getExitCode() native "Process_GetExitCode";
   /* patch */ static void _sleep(int millis) native "Process_Sleep";
   /* patch */ static int _pid(Process process) native "Process_Pid";
   /* patch */ static Stream<ProcessSignal> _watchSignal(ProcessSignal signal) {
     if (signal != ProcessSignal.SIGHUP &&
         signal != ProcessSignal.SIGINT &&
         signal != ProcessSignal.SIGTERM &&
         (Platform.isWindows ||
          (signal != ProcessSignal.SIGUSR1 &&
           signal != ProcessSignal.SIGUSR2 &&
           signal != ProcessSignal.SIGWINCH))) {
@@ skipping 395 matching lines @@
   const _ProcessResult(int this.pid,
                        int this.exitCode,
                        this.stdout,
                        this.stderr);
 
   final int pid;
   final int exitCode;
   final stdout;
   final stderr;
 }