Phase out local storage usage in PortSync code

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

Index: sdk/lib/html/templates/html/dartium/impl_LocalWindow.darttemplate
diff --git a/sdk/lib/html/templates/html/dartium/impl_LocalWindow.darttemplate b/sdk/lib/html/templates/html/dartium/impl_LocalWindow.darttemplate
index 6cedf2d4b6c302b05ce31dad24c4cc3d3143f1e5..476d8dd6946414e07e62ffefff4948b7f1d98d1b 100644
--- a/sdk/lib/html/templates/html/dartium/impl_LocalWindow.darttemplate
+++ b/sdk/lib/html/templates/html/dartium/impl_LocalWindow.darttemplate
@@ -20,7 +20,7 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
    * 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);
   }
 
@@ -31,7 +31,7 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
    */
   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