Support stacktrace and objecthistogram service commands

runtime/bin/vmservice/service_request.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 ServiceRequest {
   final List<String> pathSegments = new List<String>();
   final Map<String, String> parameters = new Map<String, String>();
   String _response;
@@ skipping 13 matching lines @@
         continue;
       }
       pathSegments.add(pathSegment);
     }
     uri.queryParameters.forEach((k, v) {
       parameters[k] = v;
     });
     return true;
   }
 
-  String toServiceCallMessage() {
-    return JSON.stringify({
-      'p': pathSegments,
-      'k': parameters.keys.toList(),
-      'v': parameters.values.toList()
-    });
+  List toServiceCallMessage() {
+    return [pathSegments, parameters.keys.toList(), parameters.values.toList()];
   }
 
   void setErrorResponse(String error) {
     _response = JSON.stringify({
-        'error': error,
+        'type': 'error',
+        'msg': error,
         'pathSegments': pathSegments,
         'parameters': parameters
     });
   }
 
   void setResponse(String response) {
     _response = response;
   }
 
 }