Prepare to treat keys mapped to null same way as missing keys in Map extensions: getOrElse, getOrPut, getOrImplicitDefault.

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

@@ -35,3 +35,15 @@ public fun <T> setOf(element: T): Set<T> = hashSetOf(element)
  * specified value.
  */
 public fun <K, V> mapOf(pair: Pair<K, V>): Map<K, V> = hashMapOf(pair)
+
+
+public inline fun <K, V> MutableMap<K, V>.getOrPut(key: K, defaultValue: () -> V): V {
+    val value = get(key)
+    return if (value == null && !containsKey(key)) {
+        val answer = defaultValue()
+        put(key, answer)
+        answer
+    } else {
+        value as V
+    }
+}