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

tests/compiler/dart2js_native/native_named_constructors3_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";
 
 // Hidden native class with factory constructors and NO static methods.
 // Regression test.
 
 @Native("A")
 class A {
   // No static methods in this class.
 
   factory A(int len) => makeA(len);
 
   factory A.fromString(String s) => makeA(s.length);
 
   // Only functions with zero parameters are allowed with "native r'...'".
   factory A.nativeConstructor() native r'return makeA(102);';
 
   foo() native;
 }
 
 makeA(v) native;
 
-void setup() native """
-// This code is all inside 'setup' and so not accessible from the global scope.
-function A(arg) { this._x = arg; }
-A.prototype.foo = function(){ return this._x; }
-makeA = function(arg) { return new A(arg); }
-self.nativeConstructor(A);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  // This code is inside 'setup' and so not accessible from the global scope.
+  function A(arg) { this._x = arg; }
+  A.prototype.foo = function(){ return this._x; };
+  makeA = function(arg) { return new A(arg); };
+  self.nativeConstructor(A);
+})()""");
+}
 
 main() {
   nativeTesting();
   setup();
   var a1 = new A(100);
   var a2 = new A.fromString('Hello');
   var a3 = new A.nativeConstructor();
 
   Expect.equals(100, a1.foo());
   Expect.equals(5, a2.foo());
   Expect.equals(102, a3.foo());
 }