Function watchClosed

lib/internal/webstreams/readablestream.js:1593–1598

(stream, promise, action)

  }

  function watchClosed(stream, promise, action) {
    if (stream[kState].state === 'closed')
      action();
    else
      PromisePrototypeThen(promise, action, () => {});
  }

  async function step() {
    if (shuttingDown) return true;