addVisibilityBlock method
Implementation
void addVisibilityBlock() {
visibilityBlocks ??= 0;
visibilityBlocks++;
//2.5.3 20151120
// Node.prototype.addVisibilityBlock = function () {
// if (this.visibilityBlocks == null) {
// this.visibilityBlocks = 0;
// }
// this.visibilityBlocks = this.visibilityBlocks + 1;
// };
}