Revert "Change some platform libraries to use `part of URI`."

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 68408a36ea8600abccb8b467e2e5b349bf0ab819..61b9863c76e2268d4ceb791cf197647ef29df48e 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 "dart:html_common";
+part of html_common;
 
 abstract class CssClassSetImpl implements CssClassSet {
   static final RegExp _validTokenRE = new RegExp(r'^\S+$');