clean up SortableSet

- remove unnecessary _isSorted flag
- remove clear-override as an empty set is "sorted"
- early return on empty sets

lib/util/SortableSet.js

@@ -6,7 +6,6 @@ module.exports = class SortableSet extends Set {
 		super(initialIterable);
 		this._sortFn = defaultSort;
 		this._lastActiveSortFn = null;
-		this._isSorted = false;
 	}
 
 	/**
@@ -15,26 +14,16 @@ module.exports = class SortableSet extends Set {
 	 */
 	add(value) {
 		this._lastActiveSortFn = null;
-		this._isSorted = false;
 		super.add(value);
 		return this;
 	}
 
-	/**
-	 * @returns {void}
-	 */
-	clear() {
-		this._lastActiveSortFn = null;
-		this._isSorted = false;
-		super.clear();
-	}
-
 	/**
 	 * @param {Function} sortFn - function to sort the set
 	 * @returns {void}
 	 */
 	sortWith(sortFn) {
-		if(this._isSorted && sortFn === this._lastActiveSortFn) {
+		if(this.size === 0 || sortFn === this._lastActiveSortFn) {
 			// already sorted - nothing to do
 			return;
 		}
@@ -45,7 +34,6 @@ module.exports = class SortableSet extends Set {
 			this.add(sortedArray[i]);
 		}
 		this._lastActiveSortFn = sortFn;
-		this._isSorted = true;
 	}
 
 	/**