anchor.diff:  }
   
   /// Adds a reference to the original dart instance to a js proxy object.
  -void addDartInstance(JsObject jsObject, dartInstance) {
  +void setDartInstance(JsObject jsObject, dartInstance) {
     assert(jsObject['__dartClass__'] == null);
     jsObject['__dartClass__'] = dartInstance;
   }
  
positive.code:         dartMap[key] = convertToDart(jsValue[key]);
        }
        _jsMapExpando[dartMap] = jsValue;
        addDartInstance(jsValue, dartMap);
        return dartMap;
      }
    } else if (jsValue is CustomEvent) {
  
negative.code:     if (newList == null) {
        newList = new JsArray.from(dartValue.map((item) => convertToJs(item)));
        _jsArrayExpando[dartValue] = newList;
        setDartInstance(newList, dartValue);
      }
      return newList;
    } else if (dartValue is Map) {
  