Make mutation methods in AbstractMutable-collections abstract rather than implement them unsupported.

js/js.libraries/src/core/collections/AbstractMutableCollection.kt

@@ -19,7 +19,7 @@ package kotlin.collections
 
 public abstract class AbstractMutableCollection<E> protected constructor() : AbstractCollection<E>(), MutableCollection<E> {
 
-    override fun add(element: E): Boolean = throw UnsupportedOperationException("Add not supported on this collection")
+    abstract override fun add(element: E): Boolean
 
     override fun remove(element: E): Boolean {
         val iterator = iterator()

js/js.libraries/src/core/collections/AbstractMutableList.kt

@@ -24,9 +24,9 @@ package kotlin.collections
 public abstract class AbstractMutableList<E> protected constructor() : AbstractMutableCollection<E>(), MutableList<E> {
     protected var modCount: Int = 0
 
-    override fun add(index: Int, element: E): Unit = throw UnsupportedOperationException("Add not supported on this list")
-    override fun removeAt(index: Int): E = throw UnsupportedOperationException("Remove not supported on this list")
-    override fun set(index: Int, element: E): E = throw UnsupportedOperationException("Set not supported on this list")
+    abstract override fun add(index: Int, element: E): Unit
+    abstract override fun removeAt(index: Int): E
+    abstract override fun set(index: Int, element: E): E
 
     override fun add(element: E): Boolean {
         add(size, element)

js/js.libraries/src/core/collections/AbstractMutableMap.kt

@@ -52,6 +52,7 @@ public abstract class AbstractMutableMap<K, V> protected constructor() : Abstrac
     override val keys: MutableSet<K> get() {
         if (_keys == null) {
             _keys = object : AbstractMutableSet<K>() {
+                override fun add(element: K): Boolean = throw UnsupportedOperationException("Add is not supported on keys")
                 override fun clear() {
                     this@AbstractMutableMap.clear()
                 }
@@ -81,9 +82,7 @@ public abstract class AbstractMutableMap<K, V> protected constructor() : Abstrac
         return _keys!!
     }
 
-    override fun put(key: K, value: V): V? {
-        throw UnsupportedOperationException("Put not supported on this map")
-    }
+    abstract override fun put(key: K, value: V): V?
 
     override fun putAll(from: Map<out K, V>) {
         for ((key, value) in from) {
@@ -95,6 +94,7 @@ public abstract class AbstractMutableMap<K, V> protected constructor() : Abstrac
     override val values: MutableCollection<V> get() {
         if (_values == null) {
             _values = object : AbstractMutableCollection<V>() {
+                override fun add(element: V): Boolean = throw UnsupportedOperationException("Add is not supported on values")
                 override fun clear() = this@AbstractMutableMap.clear()
 
                 override operator fun contains(element: V): Boolean = containsValue(element)

js/js.libraries/src/core/collections/HashMap.kt

@@ -27,6 +27,7 @@ public open class HashMap<K, V> : AbstractMutableMap<K, V> {
 
     private inner class EntrySet : AbstractMutableSet<MutableEntry<K, V>>() {
 
+        override fun add(element: MutableEntry<K, V>): Boolean = throw UnsupportedOperationException("Add is not supported on entries")
         override fun clear() {
             this@HashMap.clear()
         }

js/js.libraries/src/core/collections/LinkedHashMap.kt

@@ -80,6 +80,7 @@ public open class LinkedHashMap<K, V> : HashMap<K, V>, Map<K, V> {
             }
         }
 
+        override fun add(element: MutableEntry<K, V>): Boolean = throw UnsupportedOperationException("Add is not supported on entries")
         override fun clear() {
             this@LinkedHashMap.clear()
         }