code: function tee(input) {
    if (isStream(input)) {
      const teed = toStream(input).tee();
      teed[0][externalBuffer] = teed[1][externalBuffer] = input[externalBuffer];
      return teed;
    }
    return [slice(input), slice(input)];
  }
docs: Tee a Stream for reading it twice. The input stream can no longer be read after tee()ing.
  Reading either of the two returned streams will pull from the input stream.
  The input stream will only be canceled if both of the returned streams are canceled.
  @param {ReadableStream|Uint8array|String} input
  @returns {Array<ReadableStream|Uint8array|String>} array containing two copies of input