Introduce Common protected property AbstractMutableList.modCount #KT-57150

libraries/stdlib/common/src/kotlin/collections/AbstractMutableList.kt

@@ -11,6 +11,20 @@ package kotlin.collections
  * @param E the type of elements contained in the list. The list is invariant in its element type.
  */
 public expect abstract class AbstractMutableList<E> : MutableList<E> {
+    /**
+     * The number of times this list is structurally modified.
+     *
+     * A modification is considered to be structural if it changes the list size,
+     * or otherwise changes it in a way that iterations in progress may return incorrect results.
+     *
+     * This value can be used by iterators returned by [iterator] and [listIterator]
+     * to provide fail-fast behavoir when a concurrent modification is detected during iteration.
+     * [ConcurrentModificationException] will be thrown in this case.
+     */
+    // TODO: Should be @SinceKotlin("2.0"), see KT-64904
+    @SinceKotlin("1.9")
+    protected var modCount: Int
+
     protected constructor()
 
     /**