JsObject.jsify(object)

Recursively converts a JSON-like collection of Dart objects to a collection of JavaScript objects and returns a JsObject proxy to it.

object must be a Map or Iterable, the contents of which are also converted. Maps and Iterables are copied to a new JavaScript object. Primitives and other transferrable values are directly converted to their JavaScript type, and all other objects are proxied.

Source 

factory JsObject.jsify(object) {
  if ((object is! Map) && (object is! Iterable)) {
    throw new ArgumentError("object must be a Map or Iterable");
  }
  return _jsify(object);
}