Remove old frog-style dummy bodies from dart2js native class tests

tests/compiler/dart2js_native/native_class_inheritance4_frog_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "dart:_js_helper";
 import "package:expect/expect.dart";
 
 // Additional Dart code may be 'placed on' hidden native classes.  With
 // inheritance, the superclass method must not be reached by a call on the
 // subclass.
@@ skipping 11 matching lines @@
 @Native("B")
 class B extends A {
   var _field2;
 
   int get X => _field2;
   void set X(int x) { _field2 = x; }
 
   int method(int z) => _field2 + z;
 }
 
-A makeA() native { return new A(); }
-B makeB() native { return new B(); }
+A makeA() native;
+B makeB() native;
 
 void setup() native r"""
 function inherits(child, parent) {
   if (child.prototype.__proto__) {
     child.prototype.__proto__ = parent.prototype;
   } else {
     function tmp() {};
     tmp.prototype = parent.prototype;
     child.prototype = new tmp();
     child.prototype.constructor = child;
@@ skipping 97 matching lines @@
 }
 
 main() {
   testBasicA_dynamic();
   testBasicA_typed();
   testBasicB_dynamic();
   testBasicB_typed();
   testAB_dynamic();
   testAB_typed();
 }