Chrome Context menu using package:js in Dart

So I've managed to create a js extension for Chrome, but I have to rewrite it in Dart, using package:js. So my question is: Having this fragment in JS:

var contextMenuItem = {
"id": "selectedText",
"title": "Send",
"contexts": ["selection"]
};
chrome.contextMenus.create(contextMenuItem);

I want to use it in Dart. I do not know, however, how to create a contextMenuItem item in JS-dart, to use in this function:

 @JS('chrome.contextMenus.create()')
 external void createContext(??type?? contextMenuItem);

Do I need a factory, or something? Could someone show me an example? The examples on the internet just show simple variables like dynamic String. Thanks!