Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_mixin_multiple2_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";
 
 // Test that native classes can access methods defined only by mixins.
 
 @Native("A")
 class A {
   foo(x, [y]) => '$x;$y';
 }
 
 @Native("B")
 class B extends A with M1, M2, M3 {}
 
 class M1 {}
 
 class M2 {
   // These methods are only defined in this non-first, non-last mixin.
   plain(x) => 'P $x';
   bar(x, [y]) => '$y,$x';
 }
 
 class M3 {}
 
 makeB() native ;
 
 void setup() native """
 function B() {}
 makeB = function(){return new B;};
+
+self.nativeConstructor(B);
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   B b = makeB();
   Expect.equals('1;2', b.foo(1, 2));
   Expect.equals('2;null', b.foo(2));
   Expect.equals('P 3', b.plain(3));
   Expect.equals('100,4', b.bar(4, 100));
   Expect.equals('null,5', b.bar(5));
 }