Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_class_is_check1_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 "dart:_js_helper";
-import "package:expect/expect.dart";
+import "native_testing.dart";
 
 // Test for correct simple is-checks on hidden native classes.
 
 abstract class I {
   I read();
   write(I x);
 }
 
 // Native implementation.
 
 @Native("A")
 class A implements I {
   // The native class accepts only other native instances.
   A read() native ;
   write(A x) native ;
 }
 
 makeA() native ;
 
 void setup() native """
 // This code is all inside 'setup' and so not accesible from the global scope.
 function A(){}
 A.prototype.read = function() { return this._x; };
 A.prototype.write = function(x) { this._x = x; };
 makeA = function(){return new A};
+self.nativeConstructor(A);
 """;
 
 class B {}
 
 main() {
+  nativeTesting();
   setup();
 
   var a1 = makeA();
   var ob = new Object();
 
   Expect.isFalse(ob is I);
   Expect.isFalse(ob is A);
   Expect.isFalse(ob is B);
 
   Expect.isTrue(a1 is I);
   Expect.isTrue(a1 is A);
   Expect.isTrue(a1 is! B);
 }