Function start

tests/smoke/bun/tests/progress.smoke.test.ts:16–20

(controller)

    const fetch = async () => {
      const stream = new ReadableStream({
        start(controller) {
          controller.enqueue(new TextEncoder().encode('ab'));
          controller.enqueue(new TextEncoder().encode('cd'));
          controller.close();
        },
      });

      return new Response(stream, {