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

tests/compiler/dart2js_native/native_class_is_check1_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 "native_testing.dart";
 
 // Test for correct simple is-checks on hidden native classes.
 
 abstract class I {
   I read();
   write(I x);
 }
 
 // Native implementation.
 
 @Native("A")
 class A implements I {
   // The native class accepts only other native instances.
   A read() native;
   write(A x) native;
 }
 
 makeA() native;
 
-void setup() native """
-// This code is all inside 'setup' and so not accessible from the global scope.
-function A(){}
-A.prototype.read = function() { return this._x; };
-A.prototype.write = function(x) { this._x = x; };
-makeA = function(){return new A};
-self.nativeConstructor(A);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  // This code is inside 'setup' and so not accessible from the global scope.
+  function A(){}
+  A.prototype.read = function() { return this._x; };
+  A.prototype.write = function(x) { this._x = x; };
+  makeA = function(){return new A()};
+  self.nativeConstructor(A);
+})()""");
+}
 
 class B {}
 
 main() {
   nativeTesting();
   setup();
 
   var a1 = makeA();
   var ob = new Object();
 
   Expect.isFalse(ob is I);
   Expect.isFalse(ob is A);
   Expect.isFalse(ob is B);
 
   Expect.isTrue(a1 is I);
   Expect.isTrue(a1 is A);
   Expect.isTrue(a1 is! B);
 }