Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_field_invocation2_test.dart

 // Copyright (c) 2014, 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";
 
 @Native("NNative")
 class NNative {
   var status;
 
   var f;
 
-  ClickCounter() {
-    f = wrap(g);
-  }
-
   g(val) => "### $val ###";
 }
 
-nativeId(x) native ;
-
-void setup() native """
-nativeId = function(x) { return x; }
-""";
-
 class ClickCounter {
   var status;
 
   var f;
 
   ClickCounter() {
     f = wrap(g);
   }
 
   g(val) => "### $val ###";
 }
 
 wrap(cb) {
   return (val) {
     return cb("!!! $val !!!");
   };
 }
 
+nativeId(x) native ;
+
+void setup() native """
+nativeId = function(x) { return x; }
+""";
+
+
 main() {
   setup();
   // Make sure the ClickCounter class goes through interceptors.
   Expect.equals("### !!! 42 !!! ###", nativeId(new ClickCounter()).f(42));
   // We are interested in the direct call, where the type is known.
   Expect.equals("### !!! 499 !!! ###", new ClickCounter().f(499));
 }