Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/hash_code_test.dart

 // Copyright (c) 2012, 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';
 
 @Native("A")
 class A {}
 
+@Native("B")
+class B {
+  int get hashCode => 1234567;
+}
+
 makeA() native ;
+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);
 """;
 
 main() {
+  nativeTesting();
   setup();
   Expect.isTrue(makeA().hashCode is int);
+  Expect.equals(1234567, makeB().hashCode);
 }