anchor.diff:      var type = _stripRef(map['type']);
       if (type == 'VM') {
         // Update this VM object.
  -      update(map);
  +      updateFromServiceMap(map);
         return this;
       }
   
  
positive.code:   }
  
    /// Update [this] using [map] as a source. [map] can be a reference.
    void update(Map map) {
      assert(_isServiceMap(map));
  
      // Don't allow the type to change on an object update.
  
negative.code:     if (obj == null) {
        obj = new ServiceMap._empty(owner);
      }
      obj.updateFromServiceMap(map);
      return obj;
    }
  
  