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

tests/compiler/dart2js_native/native_testing.dart

 // Copyright (c) 2016, 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.
 
 // Native testing library. Provides support for mock @Native classes and
 // collects common imports.
 
 import "package:expect/expect.dart";
 import 'dart:_js_helper' show Native;
+import 'dart:_foreign_helper' show JS;
 
 export "package:expect/expect.dart";
 export 'dart:_js_helper' show Creates, Native, JSName, Returns;
 export 'dart:_foreign_helper' show JS;
 
-void _setup() native r'''
+void nativeTesting() {
+  JS('', r'''
 ((function() {
   var toStringResultProperty = "_toStringResult";
   var objectToStringMethod = Object.prototype.toString;
 
   Object.prototype.toString = function() {
     if (this != null) {
       var constructor = this.constructor;
       if (constructor != null) {
         var result = constructor[toStringResultProperty];
         if (typeof result == "string") return result;
       }
     }
     return objectToStringMethod.call(this);
-  }
+  };
 
   // To mock a @Native class with JavaScript constructor `Foo`, add
   //
   //     self.nativeConstructor(Foo);
   //
   // to the JavaScript code.
   self.nativeConstructor = function(constructor, opt_name) {
     var toStringResult = "[object " + (opt_name || constructor.name) + "]";
     constructor[toStringResultProperty] = toStringResult;
-  }
-})());
-''';
-
-void nativeTesting() {
-  _setup();
+  };
+})())
+''');
 }
 
 @NoInline()
 @AssumeDynamic()
 confuse(x) => x;