Not documented.

Source

convertDartToNative_Dictionary(Map dict) {
  if (dict == null) return null;
  var jsObject = new js.JsObject(js.context['Object']);
  dict.forEach((String key, value) {
    if (value is List) {
      var jsArray = new js.JsArray();
      value.forEach((elem) {
        jsArray.add(elem is Map ? convertDartToNative_Dictionary(elem): elem);
      });
      jsObject[key] = jsArray;
    } else {
      jsObject[key] = value;
    }
  });
  return jsObject;
}