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

tests/compiler/dart2js_native/native_class_fields_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";
 
 // Verify that native fields on classes are not renamed by the minifier.
 @Native("A")
 class A {
@@ skipping 22 matching lines @@
                               writeable: false
                             }
       });
   a.getValue = 0;
   return a;
 }
 
 makeA = function(){return new A;};
 """;
 
-A makeA() native { return new A(); }
+A makeA() native;
 
 main() {
   setup();
   var a = makeA();
   a.myLongPropertyName = 21;
   int gotten = a.myLongPropertyName;
   Expect.equals(11, gotten);
 
   var a2 = makeA();
   if (a2 is A) {
     // Inside this 'if' the compiler knows that a2 is an A, so it is tempted
     // to access myLongPropertyName directly, using its minified name.  But
     // renaming of native properties can only work using getters and setters
     // that access the original name.
     a2.myLongPropertyName = 21;
     int gotten = a2.myLongPropertyName;
     Expect.equals(11, gotten);
   }
 }