JS: Drop native map get/set intrinsics altogether.

MutableMap.set extension operator is now common among stdlib and stdlib-js.
#KT-2323

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

@@ -130,6 +130,14 @@ public inline operator fun <@kotlin.internal.OnlyInputTypes K, V> Map<out K, V>.
 public inline operator fun <@kotlin.internal.OnlyInputTypes K, V> Map<out K, V>.get(key: K): V?
         = @Suppress("UNCHECKED_CAST") (this as Map<K, V>).get(key)
 
+/**
+ * Allows to use the index operator for storing values in a mutable map.
+ */
+@kotlin.internal.InlineOnly
+public inline operator fun <K, V> MutableMap<K, V>.set(key: K, value: V): Unit {
+    put(key, value)
+}
+
 /**
  * Returns `true` if the map contains the specified [key].
  *

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

@@ -11,14 +11,7 @@ import java.util.SortedMap
 import java.util.TreeMap
 import java.util.concurrent.ConcurrentMap
 
-/**
- * Allows to use the index operator for storing values in a mutable map.
- */
-// this code is JVM-specific, because JS has native set function
-@kotlin.internal.InlineOnly
-public inline operator fun <K, V> MutableMap<K, V>.set(key: K, value: V): Unit {
-    put(key, value)
-}
+
 
 /**
  * Concurrent getOrPut, that is safe for concurrent maps.