Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/undefined_bailout_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.
 
 // dart2js regression test where the SSA backend would use the wrong
 // type for an instruction: when doing a speculative type propagation,
 // if an instruction gets analyzed multiple times, we used to save the
 // last inferred type, which could be speculative, instead of the
 // first inferred type, which is the actual, non-speculative, type.
 //
 // Because we are doing speculative type propagation at the very end,
 // before emitting the bailout version, the only place where we might
 // do wrong optimizations is during the codegen. It turns out that the
 // codegen optimizes '==' comparisons on null to '===' in case it knows
 // the receiver cannot be null. So in the following example, the
 // [:e == null:] comparison in [foo] got wrongly generated to a
 // JavaScript identity check (that is, using '==='). But this
 // comparison does not work for undefined, which the DOM sometimes
 // returns.
 
-import "package:expect/expect.dart";
-import 'dart:_foreign_helper' show JS;
+import "native_testing.dart";
 
 var a = 42;
 var b = 0;
 
 foo(e) {
   // Loop to force a bailout.
   for (int i = 0; i < b; i++) {
     a = e[i]; // Will desire a readable primitive.
   }
   // Our heuristics for '==' on an indexable primitive is that it's
   // more common to do it on string, so we'll want e to be a string.
   return a == e || e == null;
 }
 
 main() {
   Expect.isTrue(foo(JS('', 'void 0')));
 }