Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_mixin_multiple3_test.dart

Index: tests/compiler/dart2js_native/native_mixin_multiple3_test.dart
diff --git a/tests/compiler/dart2js_native/native_mixin_multiple3_test.dart b/tests/compiler/dart2js_native/native_mixin_multiple3_test.dart
index 3b8875ce5fb7f2d594c9180bb6e095a246cf7d44..ccbdd0cce1cb77fdb055ecca63374f441233a249 100644
--- a/tests/compiler/dart2js_native/native_mixin_multiple3_test.dart
+++ b/tests/compiler/dart2js_native/native_mixin_multiple3_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 plain classes can access methods defined only by
 // the same mixin.
@@ -46,6 +45,10 @@ function C() {}
 makeA = function(){return new A;};
 makeB = function(){return new B;};
 makeC = function(){return new C;};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
+self.nativeConstructor(C);
 """;
 
 var g;
@@ -62,6 +65,7 @@ callFoo(x) {
 makeAll() => [makeA(), makeB(), makeC(), new D(), new E()];
 
 main() {
+  nativeTesting();
   setup();
   /*
   var a = makeA();