Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_mixin_with_plain_test.dart

Index: tests/compiler/dart2js_native/native_mixin_with_plain_test.dart
diff --git a/tests/compiler/dart2js_native/native_mixin_with_plain_test.dart b/tests/compiler/dart2js_native/native_mixin_with_plain_test.dart
index 92377a10e989631638834608732a893928c43026..5311c3236be836aec60e911124cc4295a399cd4f 100644
--- a/tests/compiler/dart2js_native/native_mixin_with_plain_test.dart
+++ b/tests/compiler/dart2js_native/native_mixin_with_plain_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 and ordinary Dart classes can both use the same
 // ordinary Dart classes as a mixin.
@@ -46,9 +45,13 @@ function A() {this.aa = 'aa'}
 function B() {this.aa = 'bb'}
 makeA = function(){return new A;};
 makeB = function(){return new B;};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
 """;
 
 main() {
+  nativeTesting();
   setup();
   var things = [makeA, makeB, () => new C(), () => new D(), () => new M()]
       .map((f) => f())