Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_field_optimization_test.dart

 // Copyright (c) 2015, 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 "dart:_js_helper";
-import "package:expect/expect.dart";
+import 'native_testing.dart';
 
 // Test that compiler is cautious with optimizations on native fields.  The
 // motivation is that DOM properties are getters and setters with arbitrary
 // effects.  Setting CSSStyleDeclaration.borderLeft can canonicalize the value
 // and changes the value of CSSStyleDeclaration.border.
 
 @Native("Foo")
 class Foo {
   var a;
   var b;
@@ skipping 18 matching lines @@
 Object.defineProperty(Foo.prototype, 'ab', {
   get: function () { return this.a + ' ' + this.b; },
   set: function (v) {
     var s = v.split(' ');
     this.a = s[0];
     this.b = s[1];
   }
 });
 
 makeFoo = function() { return new Foo() }
+
+self.nativeConstructor(Foo);
 """;
 
 test1() {
   var f = makeFoo();
   f.a = 'Hi';
   f.b = 'There';
   Expect.equals('hi1 there', f.ab);
 }
 
 test2() {
@@ skipping 22 matching lines @@
 
 test4() {
   // Must not store-forward.
   var f = makeFoo();
   f.a = 'A';
   var a2 = f.a;
   Expect.equals('a1', a2);
 }
 
 main() {
+  nativeTesting();
   setup();
   (test1)();
   (test2)();
   (test3)();
   (test4)();
 }