dart2js_native tests: Migrate 'native STRING' to JS-calls

tests/compiler/dart2js_native/native_to_string_frog_test.dart

 // Copyright (c) 2012, 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 'native_testing.dart';
 
 @Native("A")
 class A {
   toString() => 'AAA';
 }
 
 makeA() native;
 
-void setup() native """
-function A() {}
-makeA = function(){return new A;};
-self.nativeConstructor(A);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {}
+  makeA = function(){return new A()};
+  self.nativeConstructor(A);
+})()""");
+}
 
 main() {
   nativeTesting();
   setup();
 
   Expect.isTrue(makeA().toString() is String);
   Expect.equals('AAA', makeA().toString());
 
   Expect.isTrue(confuse(makeA()).toString() is String);
   Expect.equals('AAA', confuse(makeA()).toString());
 }