anchor.diff:      String mapId = map['id'];
       var obj = (mapId != null) ? _cache[mapId] : null;
       if (obj != null) {
  -      obj.update(map);
  +      obj.updateFromServiceMap(map);
         return obj;
       }
       // Build the object from the map directly.
  
positive.code:   }
  
    VM() : super._empty(null) {
      update({'name': 'vm', 'type': '@VM'});
    }
  
    void postServiceEvent(String streamId, Map response, ByteData data) {
  
negative.code:     var type = _stripRef(map['type']);
      if (type == 'VM') {
        // Update this VM object.
        updateFromServiceMap(map);
        return this;
      }
  
  