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

tests/compiler/dart2js_native/native_checked_fields_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";
 
 // Test that type checks occur on assignment to fields of native methods.
 
 @Native("A")
 class A {
   int foo;
 }
 
 @Native("B")
 class B {
   String foo;
 }
 
-A makeA() native { return new A(); }
-B makeB() native { return new B(); }
+A makeA() native;
+B makeB() native;
 
 void setup() native """
 function A() {}
 
 function B() {}
 
 makeA = function(){return new A;};
 makeB = function(){return new B;};
 """;
 
@@ skipping 78 matching lines @@
 
 main() {
   setup();
 
   if (isCheckedMode()) {
     checkedModeTest();
   } else {
     uncheckedModeTest();
   }
 }