Phase out local storage usage in PortSync code

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 82eaeee52c4c6a1c846b5a213ef6f1e8bbc284d3..b95ba7970498638e686684459c63c2ff142989dd 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -12001,7 +12001,7 @@ class LocalWindow extends EventTarget implements Window native "@*DOMWindow" {
    * registered under [name].
    */
   SendPortSync lookupPort(String name) {
-    var port = JSON.parse(localStorage['dart-port:$name']);
+    var port = JSON.parse(document.documentElement.attributes['dart-port:$name']);
     return _deserialize(port);
   }
 
@@ -12012,7 +12012,7 @@ class LocalWindow extends EventTarget implements Window native "@*DOMWindow" {
    */
   void registerPort(String name, var port) {
     var serialized = _serialize(port);
-    localStorage['dart-port:$name'] = JSON.stringify(serialized);
+    document.documentElement.attributes['dart-port:$name'] = JSON.stringify(serialized);
   }