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

tests/compiler/dart2js_native/native_null_closure_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.
 
 // Test that exception unwrapping handle cases like ({foo:null}).foo().
 
 import "native_testing.dart";
 
 typedef void MyFunctionType();
 
 @Native("A")
 class A {
   setClosure(MyFunctionType f) native;
   check(MyFunctionType f) native;
   invoke() native;
 }
 
 makeA() native;
 
-void setup() native """
+void setup() {
+  JS('', r"""
+(function(){
 function A() {}
-A.prototype.setClosure = function(f) { this.f = f; };
-A.prototype.check = function(f) { return this.f === f; };
-A.prototype.invoke = function() { return this.f(); };
-makeA = function(){return new A;};
+  A.prototype.setClosure = function(f) { this.f = f; };
+  A.prototype.check = function(f) { return this.f === f; };
+  A.prototype.invoke = function() { return this.f(); };
+  makeA = function(){return new A()};
 
-self.nativeConstructor(A);
-""";
+  self.nativeConstructor(A);
+})()""");
+}
 
 main() {
   nativeTesting();
   setup();
   A a = makeA();
   a.setClosure(null);
   Expect.isTrue(a.check(null));
   bool caughtException = false;
   try {
     a.invoke();
   } on NoSuchMethodError catch (e) {
     print(e);
     caughtException = true;
   }
   Expect.isTrue(caughtException);
 }