dart - Redstone インターセプターに CORS ヘッダーを追加するにはどうすればよいですか?

Question

I'm trying to add CORS headers to incoming requests but I've noticed that app.response.headers is an immutable map and app.request.response doesn't exist even though it appears in the documentation examples. So, to the OPTIONS request I'm replying using a new Shelf response, but I can't find a way to add any new headers to the response of the actual request. Any ideas?

@app.Interceptor(r"/api/.*", chainIdx: 1)
corsInterceptor() {

    if (app.request.method == "OPTIONS") {
        var response = new shelf.Response.ok("", headers: HEADERS);
        app.chain.interrupt(statusCode: HttpStatus.OK, responseValue: response);
    } else {
    // app.request.response is not available
        app.request.response.headers.add('Access-Control-Allow-Origin', '*');
        app.chain.next();
    }
}