Fix Windows 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 97 matching lines @@
 
   // The directory to look in to resolve "package:" scheme URIs. By default it
   // is the 'packages' directory right next to the script.
   Uri _packageRoot = null;
 
   // The map describing how certain package names are mapped to Uris.
   Uri _packageConfig = null;
   Map<String, Uri> _packageMap = null;
 
   _setPackageRoot(String packageRoot) {
+    packageRoot = _sanitizeWindowsPath(packageRoot);
     packageRoot = _enforceTrailingSlash(packageRoot);
     if (packageRoot.startsWith('file:') ||
         packageRoot.startsWith('http:') ||
         packageRoot.startsWith('https:')) {
       _packageRoot = _workingDirectory.resolve(packageRoot);
     } else {
       packageRoot = _sanitizeWindowsPath(packageRoot);
       packageRoot = _trimWindowsPath(packageRoot);
       _packageRoot = _workingDirectory.resolveUri(new Uri.file(packageRoot));
     }
@@ skipping 851 matching lines @@
           }
           resolvedUri = null;
         }
         sp.send(resolvedUri);
       });
     break;
     default:
       _log('Unknown loader request tag=$tag from $isolateId');
   }
 }