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

tests/compiler/dart2js_native/native_null_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 values of some basic types.
 
 @Native("A")
 class A {
   returnNull() native;
   returnUndefined() native;
   returnEmptyString() native;
   returnZero() native;
 }
 
 A makeA() native;
 
-void setup() native """
-function A() {}
-A.prototype.returnNull = function() { return null; };
-A.prototype.returnUndefined = function() { return void 0; };
-A.prototype.returnEmptyString = function() { return ""; };
-A.prototype.returnZero = function() { return 0; };
-makeA = function(){return new A;};
-self.nativeConstructor(A);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  function A() {}
+  A.prototype.returnNull = function() { return null; };
+  A.prototype.returnUndefined = function() { return void 0; };
+  A.prototype.returnEmptyString = function() { return ""; };
+  A.prototype.returnZero = function() { return 0; };
+  makeA = function(){return new A()};
+  self.nativeConstructor(A);
+})()""");
+}
 
 @NoInline()
 staticTests() {
   A a = makeA();
   Expect.equals(null, a.returnNull());
   Expect.equals(null, a.returnUndefined());
 
   Expect.equals('', a.returnEmptyString());
   Expect.isTrue(a.returnEmptyString().isEmpty);
   Expect.isTrue(a.returnEmptyString() is String);
@@ skipping 15 matching lines @@
   Expect.isTrue(confuse(a).returnZero() is int);
   Expect.equals(0, confuse(a).returnZero());
 }
 
 main() {
   nativeTesting();
   setup();
   staticTests();
   dynamicTests();
 }