Make 'add' function abstract in AbstractMutableSet in JVM

libraries/stdlib/src/kotlin/collections/AbstractMutableSet.kt

@@ -5,6 +5,5 @@ import java.util.AbstractSet
 
 @SinceKotlin("1.1")
 public abstract class AbstractMutableSet<E> protected constructor() : MutableSet<E>, AbstractSet<E>() {
-    // nothing to make abstract
+    abstract override fun add(element: E): Boolean
-    // it's a class rather than typealias in order to have bridge for `size` generated and nice non-platform types in methods
 }

libraries/tools/binary-compatibility-validator/reference-public-api/kotlin-stdlib.txt

@@ -192,6 +192,7 @@ public abstract class kotlin/collections/AbstractMutableMap : java/util/Abstract
 
 public abstract class kotlin/collections/AbstractMutableSet : java/util/AbstractSet, java/util/Set, kotlin/jvm/internal/markers/KMutableSet {
 	protected fun <init> ()V
+	public abstract fun add (Ljava/lang/Object;)Z
 	public abstract fun getSize ()I
 	public final fun size ()I
 }