Don't use file-based part-of when part files are shared.

sdk/lib/html/html_common/css_class_set.dart

Index: sdk/lib/html/html_common/css_class_set.dart
diff --git a/sdk/lib/html/html_common/css_class_set.dart b/sdk/lib/html/html_common/css_class_set.dart
index 4f2dec16c125aa973604f0dbdda48c4a3ad29319..11c35cbb927e65d91d00a5a7edae884d0f0ff67c 100644
--- a/sdk/lib/html/html_common/css_class_set.dart
+++ b/sdk/lib/html/html_common/css_class_set.dart
@@ -2,7 +2,7 @@
 // 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 "html_common.dart";
+part of html_common;
 
 abstract class CssClassSetImpl implements CssClassSet {
   static final RegExp _validTokenRE = new RegExp(r'^\S+$');