Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_mixin_with_plain_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 and ordinary Dart classes can both use the same
 // ordinary Dart classes as a mixin.
 
 @Native("A")
 class A {
   final String aa;
   foo() => "A-foo $aa";
   baz() => "A-baz $aa";
 }
@@ skipping 22 matching lines @@
 }
 
 makeA() native ;
 makeB() native ;
 
 void setup() native """
 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())
       .toList();
   var a = things[0];
   var b = things[1];
   var c = things[2];
   var d = things[3];
   var m = things[4];
 
@@ skipping 35 matching lines @@
 
   Expect.equals("M-foo D.mm(cc)", d.foo());
   Expect.equals("D-bar -> C-baz cc", d.bar());
   Expect.equals("C-baz cc", d.baz());
   Expect.isFalse(d is A);
   Expect.isFalse(d is B);
   Expect.isTrue(d is C);
   Expect.isTrue(d is D);
   Expect.isTrue(d is M);
 }