Have mutable collection interfaces specified explicitly to get rid of platform types in parameter and return types.

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

@@ -4,6 +4,6 @@ package kotlin.collections
 import java.util.AbstractCollection
 
 @SinceKotlin("1.1")
-public abstract class AbstractMutableCollection<E> protected constructor() : AbstractCollection<E>() {
+public abstract class AbstractMutableCollection<E> protected constructor() : MutableCollection<E>, AbstractCollection<E>() {
     abstract override fun add(element: E): Boolean
 }

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

@@ -4,7 +4,7 @@ package kotlin.collections
 import java.util.AbstractList
 
 @SinceKotlin("1.1")
-public abstract class AbstractMutableList<E> protected constructor() : AbstractList<E>() {
+public abstract class AbstractMutableList<E> protected constructor() : MutableList<E>, AbstractList<E>() {
     abstract override fun set(index: Int, element: E): E
     abstract override fun removeAt(index: Int): E
     abstract override fun add(index: Int, element: E)

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

@@ -4,6 +4,6 @@ package kotlin.collections
 import java.util.AbstractMap
 
 @SinceKotlin("1.1")
-public abstract class AbstractMutableMap<K, V> protected constructor() : AbstractMap<K, V>() {
+public abstract class AbstractMutableMap<K, V> protected constructor() : MutableMap<K, V>, AbstractMap<K, V>() {
     abstract override fun put(key: K, value: V): V?
 }

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

@@ -4,6 +4,7 @@ package kotlin.collections
 import java.util.AbstractSet
 
 @SinceKotlin("1.1")
-public abstract class AbstractMutableSet<E> protected constructor() : AbstractSet<E>() {
-    // nothing to make abstract, maybe leave it typealias then?
+public abstract class AbstractMutableSet<E> protected constructor() : MutableSet<E>, AbstractSet<E>() {
+    // nothing to make abstract
+    // it's a class rather than typealias in order to have bridge for `size` generated and nice non-platform types in methods
 }