Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/subclassing_super_field_1_test.dart

Index: tests/compiler/dart2js_native/subclassing_super_field_1_test.dart
diff --git a/tests/compiler/dart2js_native/subclassing_super_field_1_test.dart b/tests/compiler/dart2js_native/subclassing_super_field_1_test.dart
index 1e602df280b05d1883ef18ad5993e6f7da405c04..c3fb6fdc5650a17e36ee53e1515427d25d5ed4a3 100644
--- a/tests/compiler/dart2js_native/subclassing_super_field_1_test.dart
+++ b/tests/compiler/dart2js_native/subclassing_super_field_1_test.dart
@@ -2,9 +2,8 @@
 // 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;
 
@@ -40,11 +39,9 @@ makeB = function(){return new B;};
 getBPrototype = function(){return B.prototype;};
 """;
 
-var inscrutable;
-
 main() {
+  nativeTesting();
   setup();
-  inscrutable = (x) => x;
 
   setNativeSubclassDispatchRecord(getBPrototype(), findInterceptorForType(B));
 
@@ -56,8 +53,8 @@ main() {
 
   print(b);
 
-  Expect.equals(222, inscrutable(b).Bfoo());
-  Expect.equals(111, inscrutable(b).Afoo());
+  Expect.equals(222, confuse(b).Bfoo());
+  Expect.equals(111, confuse(b).Afoo());
 
   Expect.equals(222, b.Bfoo());
   Expect.equals(111, b.Afoo());