title: Convert Dart object to JS object
body: <p>I would like to use JS interop to expose a set of functions/objects coded in dart. </p>
  
  <p>I know how to use js-interop to share variables with JS scripts. But I have a requirement : I can't edit dart classes as they are coming from an external library without JS dependency. </p>
  
  <p>And while I know it's possible to do something like :</p>
  
  <pre><code>@anonymous
  @JS()
  class Config {
      external bool get value;
  
      external factory Config({bool value});
  
      factory Config.fromExternalLib(ExternalConfig config) {
          return new Config(value: config.value);
      }
  }
  </code></pre>
  
  <p>I'd like to avoid that, as it requires to duplicate every classes.</p>
  
  <p>Is there any way to convert any dart object to JS object without modifying or forking the class ?</p>