close() {
    if (!this.canCloseOrEnqueue()) {
      throw new TypeError(
          'Cannot close a readable stream that has already been requested to ' +
          'be closed');
    }
    this.closeInternal();
  }
Signals that the ReadableStream should close. The ReadableStream will
actually close once all of its chunks have been read.
https://streams.spec.whatwg.org/#rs-default-controller-close
@return {void}
@override