Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/static_methods_test.dart

 // Copyright (c) 2015, 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.
 
 // Accessing static native methods names:
 //   plain declaration ->  use @Native tag as 'scope' for declared name.
 //   identifier @JSName -> use @Native tag as 'scope' for @JSName.
 //   other @JSName -> use @JSName as an expression.
 
-import "package:expect/expect.dart";
-import 'dart:_js_helper' show Native, JSName, convertDartClosureToJS;
+import 'native_testing.dart';
+import 'dart:_js_helper' show convertDartClosureToJS;
 
 typedef int Callback(String s);
 
 @Native("CC") // Tag can be different to class name.
 class AA {
   // This name is not an identifier, so completely defines how to access method.
   @JSName('CC.foo')
   static int foo(String s) native ;
 
   // This name is not an identifier, so completely defines how to access method.
@@ skipping 22 matching lines @@
 
 CC.foo = function(s) { return s.length; }
 CC.bar = function(f) { return f("Bye"); }
 CC.lepton = function(f) { return f("Lepton"); }
 CC.baryon = function(f) { return f("three quarks"); }
 
 self.CC = CC;
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   // TODO(sra): Investigate why this line is necessary to get a correctly
   // compiled convertDartClosureToJS.  Without this line, the compiler crashes.
   convertDartClosureToJS(main, 1);
 
   Expect.equals(5, AA.foo("Hello"));
 
   Expect.equals(3, AA.bar((s) => s.length));
   Expect.equals(3, AA.baz((s) => s.length));
 
   Expect.equals(6, AA.lepton((s) => s.length));
   Expect.equals(6, AA.electron((s) => s.length));
 
   Expect.equals(12, AA._baryon((s) => s.length));
   Expect.equals(12, AA.proton((s) => s.length));
   Expect.throws(() => AA.baryon((s) => s.length)); // Not defined on AA.
 }