Make dart:_vmservice a proper builtin library

runtime/vm/service/constants.dart

Index: runtime/vm/service/constants.dart
diff --git a/runtime/vm/service/constants.dart b/runtime/vm/service/constants.dart
deleted file mode 100644
index 92390e6a36d3366cd3eb582543d2194706a3c330..0000000000000000000000000000000000000000
--- a/runtime/vm/service/constants.dart
+++ /dev/null
@@ -1,12 +0,0 @@
-// Copyright (c) 2013, 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.
-
-part of vmservice;
-
-// These must be kept in sync with runtime/vm/service.cc.
-class Constants {
-  static const int SERVICE_EXIT_MESSAGE_ID = 0;
-  static const int ISOLATE_STARTUP_MESSAGE_ID = 1;
-  static const int ISOLATE_SHUTDOWN_MESSAGE_ID = 2;
-}