Fix reload bots

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;
 
 _sanitizeWindowsPath(path) {
   // For Windows we need to massage the paths a bit according to
   // http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
   //
@@ skipping 75 matching lines @@
     if (packageRootFlag != null) {
       _setPackageRoot(packageRootFlag);
     }
     // If the --packages flag was passed.
     if (packagesConfigFlag != null) {
       _setPackagesConfig(packagesConfigFlag);
     }
   }
 
   void updatePackageMap(String packagesConfigFlag) {
+    if (packagesConfigFlag == null) {
+      return;
+    }
     _packageMap = null;
     _setPackagesConfig(packagesConfigFlag);
   }
 
   void cleanup() {
     _dead = true;
     if (_packagesPort != null) {
       _packagesPort.close();
       _packagesPort = null;
     }
@@ skipping 1048 matching lines @@
         msg[2] = resolvedUri.toString();
         msg[3] = null;
         msg[4] = e.toString();
         sp.send(msg);
       }
       break;
     default:
       _log('Unknown loader request tag=$tag from $isolateId');
   }
 }