Properly split dart:_vmservice across sdk and runtime

runtime/lib/vmservice/message_router.dart

Index: runtime/lib/vmservice/message_router.dart
diff --git a/runtime/lib/vmservice/message_router.dart b/runtime/lib/vmservice/message_router.dart
deleted file mode 100644
index 6f8a1ea69d06f72c420afb60287d537eaf97a400..0000000000000000000000000000000000000000
--- a/runtime/lib/vmservice/message_router.dart
+++ /dev/null
@@ -1,9 +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;
-
-abstract class MessageRouter {
-  Future<String> route(Message message);
-}