Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/downcast_test.dart

 // Copyright (c) 2011, 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.
 
 // Test for downcasts on native classes.
 
-import "dart:_js_helper";
-import "package:expect/expect.dart";
+import "native_testing.dart";
 
 abstract class J {}
 
 abstract class I extends J {
   I read();
   write(I x);
 }
 
 // Native implementation.
 
@@ skipping 22 matching lines @@
     child.prototype.constructor = child;
   }
 }
 function A(){}
 function B(){}
 inherits(B, A);
 A.prototype.read = function() { return this._x; };
 A.prototype.write = function(x) { this._x = x; };
 makeA = function(){return new A};
 makeB = function(){return new B};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
 """;
 
 class C {}
 
 bool _check(a, b) => identical(a, b);
 
 main() {
+  nativeTesting();
   setup();
 
   var a1 = makeA();
   var b1 = makeB();
   var ob = new Object();
 
   Expect.throws(() => ob as J);
   Expect.throws(() => ob as I);
   Expect.throws(() => ob as A);
   Expect.throws(() => ob as B);
   Expect.throws(() => ob as C);
 
   // Use b1 first to prevent a1 is checks patching the A prototype.
   Expect.equals(b1, b1 as J);
   Expect.equals(b1, b1 as I);
   Expect.equals(b1, b1 as A);
   Expect.equals(b1, b1 as B);
 
   Expect.equals(a1, a1 as J);
   Expect.equals(a1, a1 as I);
   Expect.equals(a1, a1 as A);
   Expect.throws(() => a1 as B);
   Expect.throws(() => a1 as C);
 }