mask_(height) {
    e2e.assert(
        height >= 0 && height <= TREE_SIZE_, 'bad height ' + height,
        InvalidArgumentsError);
    const depth = TREE_SIZE_ - height;
    const byteDepth = depth >> 3;
    return this.indexBytes_.map(function(byte, i) {
      if (i < byteDepth) {
        return byte;
      } else if (i == byteDepth) {
        return byte & LEFTMASKS_[depth % 8];
      } else {
        return 0x00;
      }
    });
  }
Returns a byte array representing a copy of the underlying ByteArray with
the rightmost `height` bits set to 0.

@param {number} height
@return {!e2e.ByteArray}