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

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/process_set_exit_code_script.dart b/tests/standalone/io/process_set_exit_code_script.dart
index 9a34f052a14619eff51b43f7b0e620f0ee8dbf41..38a8075367ef78c083c07e02fba5224cd89e17f3 100644
--- a/tests/standalone/io/process_set_exit_code_script.dart
+++ b/tests/standalone/io/process_set_exit_code_script.dart
@@ -5,7 +5,13 @@
 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";
+  }
 }