KJS: reimplement Json.add in Kotlin

js/js.libraries/src/core/json.kt

@@ -13,8 +13,15 @@ public fun json(vararg pairs: Pair<String, Any?>): Json {
     return res
 }
 
-@library("jsonAddProperties")
-public fun Json.add(other: Json): Json = noImpl
+public fun Json.add(other: Json): Json {
+    val keys: Array<String> = js("Object").keys(other)
+    for (key in keys) {
+        if (other.asDynamic().hasOwnProperty(key)) {
+            this[key] = other[key];
+        }
+    }
+    return this
+}
 
 public external interface JsonClass {
     public fun stringify(o: Any): String

js/js.libraries/src/js/kotlin_lib.js

@@ -340,14 +340,5 @@ Kotlin.arrayFromFun = function (size, initFun) {
     return result;
 };
 
-Kotlin.jsonAddProperties = function (obj1, obj2) {
-    for (var p in obj2) {
-        if (obj2.hasOwnProperty(p)) {
-            obj1[p] = obj2[p];
-        }
-    }
-    return obj1;
-};
-
 Kotlin.identityHashCode = getObjectHashCode;