Move exit from the builtin library to dart:io.

runtime/bin/process.dart

Index: runtime/bin/process.dart
diff --git a/runtime/bin/process.dart b/runtime/bin/process.dart
index 26eebd992c98cc2fbab5c480b0528c8d39f1d989..8acb42639ca08c2f3aa9cfda2c1d309ccf2306d7 100644
--- a/runtime/bin/process.dart
+++ b/runtime/bin/process.dart
@@ -2,6 +2,14 @@
 // 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.
 
+/** Exit the Dart VM process with the given [status] code. */
+void exit(int status) {
+  if (status is !int) {
+    throw new IllegalArgumentException("int status expected");
+  }
+  _exit(status);
+}
+
 /**
  * [Process] is used to start new processes using the static
  * [start] and [run] methods.