function lineStringIsClosed(flatCoordinates, offset, end, stride) {
  const lastCoord = end - stride;
  if (
    flatCoordinates[offset] === flatCoordinates[lastCoord] &&
    flatCoordinates[offset + 1] === flatCoordinates[lastCoord + 1] &&
    (end - offset) / stride > 3
  ) {
    return !!linearRingArea(flatCoordinates, offset, end, stride);
  }
  return false;
}
Check if the linestring is a boundary.
@param {Array<number>} flatCoordinates Flat coordinates.
@param {number} offset Offset.
@param {number} end End.
@param {number} stride Stride.
@return {boolean} The linestring is a boundary.