message: Make isItemSelectedCore protected
diff: @@ -141,11 +141,7 @@ export class QuestionCheckboxModel extends QuestionCheckboxBase {
  }
  this.renderedValue = val;
  }
  - /**
  - * Returns true if item is checked
  - * @param item checkbox item value
  - */
  - public isItemSelectedCore(item: ItemValue): boolean {
  + protected isItemSelectedCore(item: ItemValue): boolean {
  if (item === this.selectAllItem) return this.isAllSelected;
  var val = this.renderedValue;
  if (!val || !Array.isArray(val)) return false;
  