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

tests/standalone/io/process_set_exit_code_script.dart

Index: tests/standalone/io/process_set_exit_code_script.dart
diff --git a/tests/standalone/io/print_env.dart b/tests/standalone/io/process_set_exit_code_script.dart
similarity index 71%
copy from tests/standalone/io/print_env.dart
copy to tests/standalone/io/process_set_exit_code_script.dart
index 997c3a802b686935d086c0ff9b1b265232e6e496..6a784298b6fc5d9d0e05f0b09bb4498cc36f5743 100644
--- a/tests/standalone/io/print_env.dart
+++ b/tests/standalone/io/process_set_exit_code_script.dart
@@ -5,5 +5,7 @@
 import "dart:io";
 
 main() {
-  print(Platform.environment[new Options().arguments[0]]);
-}
+  stdout.writeString("standard out");
+  stderr.writeString("standard error");
+  exitCode = 25;
+}