Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_field_invocation5_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";
 
 makeCC() native ;
 nativeFirst(x, y) native ;
 
 void setup() native """
 function CC() {}
 makeCC = function() { return new CC; }
 nativeFirst = function(x, y) { return x; }
+self.nativeConstructor(CC);
 """;
 
 class C {
   foo(x) => x;
 }
 
 @Native("CC")
 class ClickCounter {
   var status;
 
   var foo;
 
   init() {
     foo = wrap(g);
   }
 
   g(val) => "### $val ###";
 }
 
 wrap(cb) {
   return (val) {
     return cb("!!! $val !!!");
   };
 }
 
 main() {
+  nativeTesting();
   setup();
   var c = makeCC();
   c.init();
   var c2 = new C();
   c = nativeFirst(c, c2);
   // After the `nativeFirst` invocation dart2js doesn't know if c is a
   // ClickCounter or C. It must go through the interceptor and call the foo$1
   // invocation.
   Expect.equals("### !!! 499 !!! ###", c.foo(499));
 }