code: protected CharSequence getCallBackScript(final String _url)
      {
          final StringBuilder js = new StringBuilder()
              .append("baseWindow.withDoc(top.dojo.doc, function () {\n")
              .append("var node = dom.byId('downloadFrame');\n")
              .append("if (node == null) {\n")
              .append("node = domConstruct.place('<iframe id=\"downloadFrame\" src=\"about:blank\" ")
                  .append("style=\"position: absolute; left: 1px; top: 1px; height: 1px; width: 1px; ")
                  .append("visibility: hidden\">', win.body());\n")
              .append("}\n")
              .append("node.src='").append(_url).append("';")
              .append("});\n");
          return DojoWrapper.require(js, DojoClasses.baseWindow, DojoClasses.dom, DojoClasses.domConstruct);
      }
docs: Script that searches for an existing iframe to use it for the download of the frame.
  If it does not exist it will be created.
  
  @param _url the url
  @return the callback script