Fix dispatch on MemberSets to always go dynamic if needed.

frog/world.dart

Index: frog/world.dart
diff --git a/frog/world.dart b/frog/world.dart
index 74203297ba0f2867af2688a0ecb94dae37dd5300..ae41c6828de83d619947758374c0f248ee6f285d 100644
--- a/frog/world.dart
+++ b/frog/world.dart
@@ -152,7 +152,7 @@ class World {
 
     var mset = _members[member.name];
     if (mset == null) {
-      mset = new MemberSet(member);
+      mset = new MemberSet(member, isVar:true);
       _members[mset.name] = mset;
     } else {
       mset.members.add(member);