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

tests/compiler/dart2js_native/native_constructor_name_test.dart

 // Copyright (c) 2013, 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 to see if a Dart class name is confused with dispatch tag.  Both class A
 // and class Z have a JavaScript constructor named "A".  The dynamic native
 // class dispatch hook on Object.prototype should avoid patching the Dart class
 // A.  This could be done by renaming the Dart constructor or by being able to
 // check that objects are Dart classes.
 
 import "native_testing.dart";
 
 class A {}
 
 @Native("A")
 class Z {
   foo() => 100;
 }
 
 makeZ() native;
 
-void setup() native """
-function A(){}
-makeZ = function(){return new A};
-self.nativeConstructor(A);
-""";
+void setup() {
+  JS('', r"""
+(function(){
+  function A(){}
+  makeZ = function(){return new A()};
+  self.nativeConstructor(A);
+})()""");
+}
 
 main() {
   nativeTesting();
   setup();
 
   var a = new A();
   var z = makeZ();
 
   Expect.equals(100, z.foo());
 
   Expect.throws(() => a.foo(), (ex) => ex is NoSuchMethodError);
 }