Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_checked_fields_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 that type checks occur on assignment to fields of native methods.
 
 @Native("A")
 class A {
   int foo;
 }
 
 @Native("B")
 class B {
   String foo;
 }
 
 A makeA() native ;
 B makeB() native ;
 
 void setup() native """
 function A() {}
 
 function B() {}
 
 makeA = function(){return new A;};
 makeB = function(){return new B;};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
 """;
 
 expectThrows(action()) {
   bool threw = false;
   try {
     action();
   } catch (e) {
     threw = true;
   }
   Expect.isTrue(threw);
@@ skipping 62 matching lines @@
   try {
     String s = a;
     return false;
   } catch (e) {
     // Ignore.
   }
   return true;
 }
 
 main() {
+  nativeTesting();
   setup();
 
   if (isCheckedMode()) {
     checkedModeTest();
   } else {
     uncheckedModeTest();
   }
 }