Maps and sets: cleanup, simplify, refactor.

#KT-12386

js/js.translator/testData/kotlin_lib.js

@@ -902,6 +902,10 @@
         return new Kotlin.ArrayIterator(array);
     };
 
+    Kotlin.deleteProperty = function (object, property) {
+        delete object[property];
+    };
+
     Kotlin.jsonAddProperties = function (obj1, obj2) {
         for (var p in obj2) {
             if (obj2.hasOwnProperty(p)) {