Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_equals_frog_test.dart

 // Copyright (c) 2013, 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 {}
 
+int calls = 0;
+@Native("B")
+class B {
+  bool operator==(other) {
+    ++calls;
+    return other is B;
+  }
+  int get hashCode => 1;
+}
+
 makeA() native ;
+makeB() native ;
 
 void setup() native """
 function A() {}
+function B() {}
 makeA = function(){return new A;};
+makeB = function(){return new B;};
+
+self.nativeConstructor(B);
 """;
 
 main() {
+  nativeTesting();
   setup();
   var a = makeA();
   Expect.isTrue(a == a);
   Expect.isTrue(identical(a, a));
 
   Expect.isFalse(a == makeA());
   Expect.isFalse(identical(a, makeA()));
+
+  var b = makeB();
+  Expect.isTrue(b == b);
+  Expect.isTrue(identical(b, b));
+
+  Expect.isTrue(b == makeB());
+  Expect.isFalse(identical(b, makeB()));
+
+  Expect.equals(2, calls);
 }