code: public static CustomFlowPanel adoptExternal(ComplexPanel parentPanel, Element externalChild) {
          if (parentPanel == null || externalChild == null) return null;
          CustomFlowPanel rslt = new CustomFlowPanel(externalChild);
          WidgetCollection children = getChildren(parentPanel);
          children.add(rslt); // Logical attach
          adoptChild(parentPanel, rslt); // Adopt
          return rslt;
      }
docs: The same as ComplexPanel.add(child, container), but without physical DOM.appendChild() attach.
  
  @param parentPanel - depends on externalChild placement in DOM, in most of the cases it should be RootPanel.get()