JS: fix copying functions with default arguments across interfaces

Copy function to super interface first, then copy from interface
to class. Add interface set to JsClassModel for this purpose.

See KT-20625

js/js.ast/src/org/jetbrains/kotlin/js/backend/ast/JsClassModel.kt

@@ -17,5 +17,6 @@
 package org.jetbrains.kotlin.js.backend.ast
 
 class JsClassModel(val name: JsName, val superName: JsName?) {
+    val interfaces: MutableSet<JsName> = mutableSetOf()
     val postDeclarationBlock = JsGlobalBlock()
 }