Ensure constructors use the current super class.

This is an extension of 324a4a1b22.

lib/6to5/transformation/templates/class-super-constructor-call.js

@@ -1,3 +1,3 @@
-if (SUPER_NAME !== null) {
-  SUPER_NAME.apply(this, arguments);
+if (Object.getPrototypeOf(CLASS_NAME) !== null) {
+  Object.getPrototypeOf(CLASS_NAME).apply(this, arguments);
 }

lib/6to5/transformation/transformers/es6-classes.js

@@ -149,7 +149,7 @@ Class.prototype.buildBody = function () {
 
   if (!this.hasConstructor && superName && !t.isFalsyExpression(superName)) {
     constructor.body.body.push(util.template("class-super-constructor-call", {
-      SUPER_NAME: superName
+      CLASS_NAME: className
     }, true));
   }