Phase out local storage usage in PortSync code

sdk/lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate

Index: sdk/lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate
diff --git a/sdk/lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate b/sdk/lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate
index 8f095cb4bdcfae1e4dce9fcafbecc9f9e412d8e8..5542bf872f0b72cf0ba000b9694870d92e4c0010 100644
--- a/sdk/lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate
+++ b/sdk/lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate
@@ -131,7 +131,7 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS native "@*DOMWindow" {
    * registered under [name].
    */
   SendPortSync lookupPort(String name) {
-    var port = JSON.parse(localStorage['dart-port:$name']);
+    var port = JSON.parse(document.documentElement.attributes['dart-port:$name']);
     return _deserialize(port);
   }
 
@@ -142,7 +142,7 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS native "@*DOMWindow" {
    */
   void registerPort(String name, var port) {
     var serialized = _serialize(port);
-    localStorage['dart-port:$name'] = JSON.stringify(serialized);
+    document.documentElement.attributes['dart-port:$name'] = JSON.stringify(serialized);
   }
 
 $!MEMBERS