Properly split dart:_vmservice across sdk and runtime

sdk/lib/vmservice/message.dart

 // 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;
 
 class Message {
   final Completer _completer = new Completer.sync();
   bool get completed => _completer.isCompleted;
   /// Future of response.
@@ skipping 127 matching lines @@
   void setResponse(String response) {
     _completer.complete(response);
   }
 
   void setErrorResponse(int code, String details) {
     _completer.complete(encodeRpcError(this, code,
                                        details: '$method: $details'));
   }
 }
 
-bool sendIsolateServiceMessage(SendPort sp, List m)
-    native "VMService_SendIsolateServiceMessage";
-
-void sendRootServiceMessage(List m)
-    native "VMService_SendRootServiceMessage";
+external bool sendIsolateServiceMessage(SendPort sp, List m);
+external void sendRootServiceMessage(List m);