Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_mixin_test.dart

Index: tests/compiler/dart2js_native/native_mixin_test.dart
diff --git a/tests/compiler/dart2js_native/native_mixin_test.dart b/tests/compiler/dart2js_native/native_mixin_test.dart
index 667da82e685129a8b1d3121db8c7a43ce36ab153..a0ab15bc2ad09a3c05b58c91bd4bde1c69ca8002 100644
--- a/tests/compiler/dart2js_native/native_mixin_test.dart
+++ b/tests/compiler/dart2js_native/native_mixin_test.dart
@@ -2,8 +2,7 @@
 // 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 native classes can use ordinary Dart classes as mixins.
 
@@ -31,9 +30,13 @@ function A() {}
 function B() {}
 makeA = function(){return new A;};
 makeB = function(){return new B;};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
 """;
 
 main() {
+  nativeTesting();
   setup();
   A a = makeA();
   Expect.equals("A-foo", a.foo());