When no service response is requested, send null back to the service isolate so it can cleanup

runtime/vm/service/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 76 matching lines @@
       }
       list[i] = list[i].toString();
     }
     return list;
   }
 
   Future<String> send(SendPort sendPort) {
     final receivePort = new RawReceivePort();
     receivePort.handler = (value) {
       receivePort.close();
-      assert(value is String);
       _completer.complete(value);
     };
     var keys = _makeAllString(params.keys.toList(growable:false));
     var values = _makeAllString(params.values.toList(growable:false));
     var request = new List(6)
         ..[0] = 0  // Make room for OOB message type.
         ..[1] = receivePort.sendPort
         ..[2] = serial
         ..[3] = method
         ..[4] = keys
         ..[5] = values;
     if (!sendIsolateServiceMessage(sendPort, request)) {
       _completer.complete(JSON.encode({
           'type': 'ServiceError',
           'id': '',
           'kind': 'InternalError',
           'message': 'could not send message [${serial}] to isolate',
       }));
     }
     return _completer.future;
   }
 
   Future<String> sendToVM() {
     final receivePort = new RawReceivePort();
     receivePort.handler = (value) {
       receivePort.close();
-      assert(value is String);
       _completer.complete(value);
     };
     var keys = _makeAllString(params.keys.toList(growable:false));
     var values = _makeAllString(params.values.toList(growable:false));
     var request = new List(6)
         ..[0] = 0  // Make room for OOB message type.
         ..[1] = receivePort.sendPort
         ..[2] = serial
         ..[3] = method
         ..[4] = keys
@@ skipping 21 matching lines @@
     };
     _completer.complete(JSON.encode(response));
   }
 }
 
 bool sendIsolateServiceMessage(SendPort sp, List m)
     native "VMService_SendIsolateServiceMessage";
 
 void sendRootServiceMessage(List m)
     native "VMService_SendRootServiceMessage";