JS backend: use javascript Object as base class for object by default (when there are not any parents).

It's workaround for compatibility with Google Chrome Extension API because sometimes they calls Object.prototype methods.

(cherry picked from commit 9959a1e)

js/js.translator/testFiles/kotlin_lib_ecma5.js

@@ -72,7 +72,7 @@ var Kotlin = Object.create(null);
 
     function computeProto2(bases, properties) {
         if (bases === null) {
-            return null;
+            return Object.prototype;
         }
         return Array.isArray(bases) ? computeProto(bases, properties) : bases.proto;
     }