Fix how we compute _packageRoot in loader.dart. This is to fix a rare

pkg/polymer/lib/src/loader.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 polymer;
 
 /** Annotation used to automatically register polymer elements. */
 class CustomTag {
   final String tagName;
   const CustomTag(this.tagName);
@@ skipping 121 matching lines @@
 }
 
 /** All libraries in the current isolate. */
 final _libs = currentMirrorSystem().libraries;
 
 // TODO(sigmund): explore other (cheaper) ways to resolve URIs relative to the
 // root library (see dartbug.com/12612)
 final _rootUri = currentMirrorSystem().isolate.rootLibrary.uri;
 
 final String _packageRoot =
-    '${path.dirname(Uri.parse(window.location.href).path)}/packages/';
+    path.url.join(path.url.dirname(Uri.parse(window.location.href).path),
+        'packages') + '/';
 
 final Logger _loaderLog = new Logger('polymer.loader');
 
 /**
  * Reads the library at [uriString] (which can be an absolute URI or a relative
  * URI from the root library), and:
  *
  *   * If present, invokes any top-level and static functions marked
  *     with the [initMethod] annotation (in the order they appear).
  *
@@ skipping 58 matching lines @@
     print("warning: methods marked with @initMethod should take no "
         "arguments, ${method.simpleName} expects some.");
     return;
   }
   obj.invoke(method.simpleName, const []);
 }
 
 class _InitMethodAnnotation {
   const _InitMethodAnnotation();
 }