Added builtin native call to exit the running vm

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index c54c48619e6e1e85bbf3fc9a01e75f49887b8911..adc1c636d1b0c6cd01d6aea2276b20154954af51 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -8,6 +8,15 @@ void print(arg) {
   _Logger._printString(arg.toString());
 }
 
+void exit(int status) {
+  if (status is !int) {
+    throw new IllegalArgumentException("int status expected");
+  }
+  _exit(status);
+}
+
+_exit(int status) native "Exit";
+
 class _Logger {
   static void _printString(String s) native "Logger_PrintString";
 }