get body () {
    if (!this[kBody]) {
      this[kBody] = ReadableStreamFrom(this)
      if (this[kConsume]) {
        // TODO: Is this the best way to force a lock?
        this[kBody].getReader() // Ensure stream is locked.
        assert(this[kBody].locked)
      }
    }
    return this[kBody]
  }
@see https://fetch.spec.whatwg.org/#dom-body-body
@readonly
@returns {ReadableStream}