Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_class_inheritance4_frog_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.
 
-import "dart:_js_helper";
-import "package:expect/expect.dart";
+import "native_testing.dart";
 
 // Additional Dart code may be 'placed on' hidden native classes.  With
 // inheritance, the superclass method must not be reached by a call on the
 // subclass.
 
 @Native("A")
 class A {
   var _field;
 
   int get X => _field;
@@ skipping 29 matching lines @@
     child.prototype = new tmp();
     child.prototype.constructor = child;
   }
 }
 
 function A(){}
 function B(){}
 inherits(B, A);
 makeA = function(){return new A;};
 makeB = function(){return new B;};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
 """;
 
 testBasicA_dynamic() {
   setup(); // Fresh constructors.
 
   var a = [makeA()][0];
 
   a.X = 100;
   Expect.equals(100, a._field);
   Expect.equals(100, a.X);
@@ skipping 72 matching lines @@
   b._field = 1;
   b._field2 = 2;
   b.X = 123;
   Expect.equals(1, b._field);
   Expect.equals(123, b._field2);
   Expect.equals(123, b.X);
   Expect.equals(200, b.method(77));
 }
 
 main() {
+  nativeTesting();
   testBasicA_dynamic();
   testBasicA_typed();
   testBasicB_dynamic();
   testBasicB_typed();
   testAB_dynamic();
   testAB_typed();
 }