Move exit from the builtin library to dart:io.

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index 80940135fba046a1a95550d940a357b277e863af..d7f5974f96f8ef78caf40374e8b8e9d4e5cad60a 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -9,15 +9,6 @@ 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";
 }