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

tests/compiler/dart2js_native/native_named_constructors2_frog_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";
 
 // Native class with named constructors and static methods.
 
 
 
-class A native "A" {
+@Native("A")
+class A {
 
   factory A(int len) => _construct(len);
 
   factory A.fromString(String s)  => _construct(s.length);
 
   // Only functions with zero parameters are allowed with "native r'...'".
   factory A.nativeConstructor() native r'return makeA(102);';
 
   static A _construct(v) { return makeA(v); }
 
@@ skipping 12 matching lines @@
 main() {
   setup();
   var a1 = new A(100);
   var a2 = new A.fromString('Hello');
   var a3 = new A.nativeConstructor();
 
   Expect.equals(100, a1.foo());
   Expect.equals(5, a2.foo());
   Expect.equals(102, a3.foo());
 }