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

tests/compiler/dart2js_native/native_closure_identity_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';
 
 typedef void MyFunctionType();
 
 @Native("A")
 class A {
   setClosure(MyFunctionType f) native;
   check(MyFunctionType f) native;
   invoke() native;
 }
 
 makeA() native;
 
-void setup() native """
-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;};
+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()};
 
-self.nativeConstructor(A);
-""";
+  self.nativeConstructor(A);
+})()""");
+}
 
 var staticClosure;
 staticMethod() => 42;
 
 class B {
   var instanceClosure;
   instanceMethod() => 43;
 }
 
 checkUntyped(a, closure) {
@@ skipping 30 matching lines @@
 
   checkUntyped(makeA(), b.instanceMethod);
   checkTyped(makeA(), b.instanceMethod);
 
   checkUntyped(makeA(), closureStatement);
   checkTyped(makeA(), closureStatement);
 
   checkUntyped(makeA(), closureExpression);
   checkTyped(makeA(), closureExpression);
 }