Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_no_such_method_exception2_frog_test.dart

 // Copyright (c) 2012, 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";
 
 @Native("A")
 class A {}
 
 @Native("B")
 class B extends A {
   foo() native ;
 }
 
 makeA() native ;
 makeB() native ;
 
 setup() native """
 function inherits(child, parent) {
   if (child.prototype.__proto__) {
     child.prototype.__proto__ = parent.prototype;
   } else {
     function tmp() {};
     tmp.prototype = parent.prototype;
     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; }
   B.prototype.foo = function() { return 42; }
+
+  self.nativeConstructor(A);
+  self.nativeConstructor(B);
 """;
 
 main() {
+  nativeTesting();
   setup();
   var a = makeA();
   var exception;
   try {
     a.foo();
   } on NoSuchMethodError catch (e) {
     exception = e;
   }
   Expect.isNotNull(exception);
 
   var b = makeB();
   Expect.equals(42, b.foo());
 }