Provide protected toArray implementation in AbstractCollection.

Relates to #KT-13898

libraries/stdlib/test/collections/CollectionTest.kt

@@ -905,4 +905,30 @@ class CollectionTest {
         assertTrue(listOf(1) is RandomAccess, "Default singleton list is RandomAccess")
         assertTrue(emptyList<Int>() is RandomAccess, "Empty list is RandomAccess")
     }
+
+    @Test fun abstractCollectionToArray() {
+        class TestCollection<out E>(val data: Collection<E>) : AbstractCollection<E>() {
+            val invocations = mutableListOf<String>()
+            override val size get() = data.size
+            override fun iterator() = data.iterator()
+
+            override fun toArray(): Array<Any?> {
+                invocations += "toArray1"
+                return data.toTypedArray()
+            }
+            public override fun <T> toArray(array: Array<T>): Array<T> {
+                invocations += "toArray2"
+                return super.toArray(array)
+            }
+        }
+        val data = listOf("abc", "def")
+        val coll = TestCollection(data)
+
+        val arr1 = coll.toTypedArray()
+        assertEquals(data, arr1.asList())
+        assertTrue("toArray1" in coll.invocations || "toArray2" in coll.invocations)
+
+        val arr2: Array<String> = coll.toArray(Array(coll.size + 1) { "" })
+        assertEquals(data + listOf(null), arr2.asList())
+    }
 }