Phase out local storage usage in PortSync code

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 6bdad865646150b50ac75557d16a6c9abd4c66e4..22d8fc097d58aa6b5b5838e116d346a7a08cfcb8 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -15385,7 +15385,7 @@ class LocalWindow extends EventTarget implements Window {
    * registered under [name].
    */
   lookupPort(String name) {
-    var port = JSON.parse(localStorage['dart-port:$name']);
+    var port = JSON.parse(document.documentElement.attributes['dart-port:$name']);
     return _deserialize(port);
   }
 
@@ -15396,7 +15396,7 @@ class LocalWindow extends EventTarget implements Window {
    */
   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);
   }
 
   LocalWindow.internal(): super.internal();