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

sdk/lib/_internal/lib/io_patch.dart

Index: sdk/lib/_internal/lib/io_patch.dart
diff --git a/sdk/lib/_internal/lib/io_patch.dart b/sdk/lib/_internal/lib/io_patch.dart
index 0bd05440c24230ff9d8e8ff2b5b38e0d545d0cc2..6e0821a5f6c5f06a8682641f420600b50ddd3af4 100644
--- a/sdk/lib/_internal/lib/io_patch.dart
+++ b/sdk/lib/_internal/lib/io_patch.dart
@@ -179,6 +179,9 @@ patch class _ProcessUtils {
   patch static void _setExitCode(int status) {
     throw new UnsupportedError("ProcessUtils._setExitCode");
   }
+  patch static int _getExitCode() {
+    throw new UnsupportedError("ProcessUtils._getExitCode");
+  }
   patch static void _sleep(int millis) {
     throw new UnsupportedError("ProcessUtils._sleep");
   }