Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/subclassing_super_field_2_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 "package:expect/expect.dart";
-import 'dart:_foreign_helper' show JS;
-import 'dart:_js_helper' show Native, Creates, setNativeSubclassDispatchRecord;
+import "native_testing.dart";
+import 'dart:_js_helper' show setNativeSubclassDispatchRecord;
 import 'dart:_interceptors'
     show findInterceptorForType, findConstructorForNativeSubclassType;
 
 // Test for fields with same name as native fields.  We expect N.foo to have the
 // property name 'foo' and A.foo and B.foo to have non-conflicting names.
 
 @Native("N")
 class N {
   var foo;
   N.init();
@@ skipping 19 matching lines @@
 @Creates('=Object')
 getBPrototype() native ;
 
 void setup() native r"""
 function B() { this.foo = 111; }  // N.foo
 makeB = function(){return new B;};
 
 getBPrototype = function(){return B.prototype;};
 """;
 
-var inscrutable;
-
 main() {
+  nativeTesting();
   setup();
-  inscrutable = (x) => x;
 
   setNativeSubclassDispatchRecord(getBPrototype(), findInterceptorForType(B));
 
   B b = makeB();
 
   var constructor = findConstructorForNativeSubclassType(B, 'init');
   Expect.isNotNull(constructor);
   JS('', '#(#)', constructor, b);
 
   print(b);
 
-  Expect.equals(333, inscrutable(b).Bfoo());
-  Expect.equals(222, inscrutable(b).Afoo());
-  Expect.equals(111, inscrutable(b).Nfoo());
+  Expect.equals(333, confuse(b).Bfoo());
+  Expect.equals(222, confuse(b).Afoo());
+  Expect.equals(111, confuse(b).Nfoo());
 
   Expect.equals(333, b.Bfoo());
   Expect.equals(222, b.Afoo());
   Expect.equals(111, b.Nfoo());
 }