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

tests/compiler/dart2js_native/native_property_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.
 
 // Properties on hidden native classes.
 
 import 'native_testing.dart';
 
 @Native("A")
 class A {
 
   // Setters and getters should be similar to these methods:
   int getX() => JS('int', '#._x', this);
   void setX(int value) { JS('void', '#._x = #', this, value); }
 
   int get X() native;
   set X(int value) native;
 
   int get Y() native;
   set Y(int value) native;
 
   int get Z => JS('int', '#._z', this);
   set Z(int value) { JS('void', '#._z = #', this, value); }
 }
 
 A makeA() native;
 
-void setup() native """
-function A() {}
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {}
 
-Object.defineProperty(A.prototype, "X", {
-  get: function () { return this._x; },
-  set: function (v) { this._x = v; }
-});
+  Object.defineProperty(A.prototype, "X", {
+    get: function () { return this._x; },
+    set: function (v) { this._x = v; }
+  });
 
-makeA = function(){return new A;};
+  makeA = function(){return new A()};
 
-self.nativeConstructor(A);
-""";
+  self.nativeConstructor(A);
+})()""");
+}
 
 
 main() {
   nativeTesting();
   setup();
 
   var a = makeA();
 
   a.setX(5);
   Expect.equals(5, a.getX());
 
   a.X = 10;
   a.Y = 20;
   a.Z = 30;
 
   Expect.equals(10, a.X);
   Expect.equals(20, a.Y);
   Expect.equals(30, a.Z);
 
   confuse(a).setX(6);
   Expect.equals(6, confuse(a).getX());
 
   Expect.equals(6, confuse(a).X);
   Expect.equals(20, confuse(a).Y);
   Expect.equals(30, confuse(a).Z);
 }