Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_mixin_multiple2_test.dart

Index: tests/compiler/dart2js_native/native_mixin_multiple2_test.dart
diff --git a/tests/compiler/dart2js_native/native_mixin_multiple2_test.dart b/tests/compiler/dart2js_native/native_mixin_multiple2_test.dart
index e3196e17e2b19c6088847472b9a771543023cd1a..2fb2b2b6d25061b4fdfb1b5da4d9d40858c4b399 100644
--- a/tests/compiler/dart2js_native/native_mixin_multiple2_test.dart
+++ b/tests/compiler/dart2js_native/native_mixin_multiple2_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 access methods defined only by mixins.
 
@@ -30,9 +29,12 @@ makeB() native ;
 void setup() native """
 function B() {}
 makeB = function(){return new B;};
+
+self.nativeConstructor(B);
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   B b = makeB();