Add @Native(...) annotation for native class names.

tests/compiler/dart2js_native/native_class_fields_test.dart

 // Copyright (c) 2011, 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";
 
 // Verify that native fields on classes are not renamed by the minifier.
-class A native "A" {
+@Native("A")
+class A {
   int myLongPropertyName;
   int getValue;
 
   int method(int z) => myLongPropertyName;
 }
 
 
 void setup() native r"""
 function getter() {
   return ++this.getValue;
 }
 
 function setter(x) {
   this.getValue += 10;
 }
-  
+
 function A(){
   var a = Object.create(
       { constructor: { name: 'A'}},
       { myLongPropertyName: { get: getter,
                               set: setter,
                               configurable: false,
                               writeable: false
                             }
       });
   a.getValue = 0;
@@ skipping 16 matching lines @@
   if (a2 is A) {
     // Inside this 'if' the compiler knows that a2 is an A, so it is tempted
     // to access myLongPropertyName directly, using its minified name.  But
     // renaming of native properties can only work using getters and setters
     // that access the original name.
     a2.myLongPropertyName = 21;
     int gotten = a2.myLongPropertyName;
     Expect.equals(11, gotten);
   }
 }