function createSuccessStreamingResponse() {
  const headers = new Headers();
  headers.set('dummyHeader', 'dummyHeaderValue');
  headers.set('dummyHeader2', 'dummyHeaderValue2');
  const bytes = new TextEncoder().encode('responseBody');
  const body = new ReadableStream({
    pull(controller) {
      for (let i = 0; i < bytes.length; i++) {
        controller.enqueue(bytes.slice(i, i + 1));
      }
      controller.close();
    },
  });
  return new Response(body, {status: 200, statusText: 'OK', headers: headers});
}
Creates a successful streaming response which returns each letter a separate
chunk with a 10ms delay between them.
@return {!Response}