Fix package load error reporting.

runtime/bin/vmservice/loader.dart

 // Copyright (c) 2015, 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_io;
 
 var _httpClient;
 
 // Send a response to the requesting isolate.
 void _sendResponse(SendPort sp, int id, dynamic data) {
@@ skipping 83 matching lines @@
   int id = request[1];
   String resource = request[2];
   var uri = Uri.parse(request[2]);
   if (uri.scheme == 'file') {
     _loadFile(sp, id, uri);
   } else if ((uri.scheme == 'http') || (uri.scheme == 'https')) {
     _loadHttp(sp, id, uri);
   } else if ((uri.scheme == 'data')) {
     _loadDataUri(sp, id, uri);
   } else {
-    sp.send('Unknown scheme (${uri.scheme}) for $uri');
+    sp.send([id, 'Unknown scheme (${uri.scheme}) for $uri']);
   }
 }