Fix dispatch on MemberSets to always go dynamic if needed.

frog/type.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class Type implements Named, Hashable {
   final String name;
   bool isTested;
 
   /**
    * For core types (int, String, etc) this is the generated type assertion
@@ skipping 819 matching lines @@
           }
           parent = resolveType(extendsTypeRef, true);
           if (!parent.isClass) {
             world.error('class may not extend an interface - use implements',
               typeDef.extendsTypes[0].span);
           }
           parent.addDirectSubtype(this);
           if (_cycleInClassExtends()) {
             world.error('class "$name" has a cycle in its inheritance chain',
                 extendsTypeRef.span);