Change JS AbstractMutableCollection.toJSON() visibility modifier from open to protected

libraries/stdlib/js/src/kotlin/collections/AbstractMutableCollection.kt

@@ -54,8 +54,9 @@ public actual abstract class AbstractMutableCollection<E> protected actual const
         }
     }
 
+    @Deprecated("Provided so that subclasses inherit this function", level = DeprecationLevel.HIDDEN)
     @JsName("toJSON")
-    open fun toJSON(): Any = this.toArray()
+    protected fun toJSON(): Any = this.toArray()
 
 
     /**

libraries/stdlib/js/test/collections/CollectionsJsTest.kt

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package test.collections.js
+
+import kotlin.test.*
+
+class CollectionsJsTest {
+    @Test
+    fun toJSON() {
+        val list = arrayListOf("array", "List", "Of")
+        assertContentEquals(arrayOf("array", "List", "Of"), list.asDynamic().toJSON() as Array<String>)
+
+        val set = linkedSetOf("linked", "Set", "Of")
+        assertContentEquals(arrayOf("linked", "Set", "Of"), (set as AbstractMutableCollection<String>).asDynamic().toJSON() as Array<String>)
+    }
+}