Added builtin native call to exit the running vm

runtime/bin/builtin.dart

 // Copyright (c) 2011, 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.
 
 #library("builtin");
 
 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";
 }