Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/subclassing_1_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.
 
-import "package:expect/expect.dart";
-import 'dart:_js_helper' show Native, Creates, setNativeSubclassDispatchRecord;
+import "native_testing.dart";
+import 'dart:_js_helper' show setNativeSubclassDispatchRecord;
 import 'dart:_interceptors' show findInterceptorForType;
 
 // Test that subclasses of native classes can be defined by setting the dispatch
 // record.
 
 @Native("A")
 class A {
   foo(x) => '$x,${this.oof()}';
   oof() => 'A';
 }
@@ skipping 19 matching lines @@
 makeA = function(){return new A;};
 makeB1 = function(){return new B;};
 makeB2 = function(){return new B;};
 makeC = function(){return new C;};
 
 getBPrototype = function(){return B.prototype;};
 getCPrototype = function(){return C.prototype;};
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   setNativeSubclassDispatchRecord(getBPrototype(), findInterceptorForType(B));
   setNativeSubclassDispatchRecord(getCPrototype(), findInterceptorForType(B));
 
   B b1 = makeB1();
   Expect.equals('1,B', b1.foo(1));
 
   B b2 = makeB2();
   Expect.equals('2,B', b2.foo(2));
 
   B b3 = makeC();
   Expect.equals('3,B', b3.foo(3));
 }