How can I flush a stream to a response?

const encoder = new TextEncoder();
Deno.serve((req) => {
  let interval: number;
  let i = 0;
  const sep = decodeURIComponent(new URL(req.url).pathname.slice(1))
  return new Response(
    new ReadableStream({
      start(c) {
        interval = setInterval(() => c.enqueue(encoder.encode(i++ + sep)), 1000)
      },
      cancel() {
        clearInterval(interval)
      }
    })
  )
})