Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_exceptions1_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';
 
 // Test that hidden native exception classes can be marked as existing.
 //
 // To infer which native hidden types exist, we need
 //   (1) return types of native methods and getters
 //   (2) argument types of callbacks
 //   (3) exceptions thrown by the operation.
 //
 // (1) and (2) can be achieved by having nicely typed native methods, but there
 // is no place in the Dart language to communicate (3).  So we use the following
@@ skipping 33 matching lines @@
 void setup2() native """
 // This code is all inside 'setup2' and so not accesible from the global scope.
 function E(x){ this.code = x; }
 
 function A(){}
 A.prototype.op = function (x) {
   if (x & 1) throw new E(100);
   return  x / 2;
 };
 makeA = function(){return new A};
+
+self.nativeConstructor(E);
+self.nativeConstructor(A);
 """;
 
 int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
 
 main() {
+  nativeTesting();
   setup1();
   setup2();
 
   var things = [makeA(), new B()];
   var a = things[inscrutable(0)];
   var b = things[inscrutable(1)];
 
   Expect.equals(25, a.op(50));
   Expect.equals(123, b.op('hello'));
   Expect.equals(666, b.code);
@@ skipping 19 matching lines @@
   threw = false;
   try {
     var x = aa.op(51);
   } on E catch (e) {
     threw = true;
     Expect.equals(100, e.code);
     Expect.isTrue(e is E);
   }
   Expect.isTrue(threw);
 }