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

tests/compiler/dart2js_native/native_mixin_multiple2_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 "dart:_js_helper";
 import "package:expect/expect.dart";
 
 // Test that native classes can access methods defined only by mixins.
 
-class A native "A" {
+@Native("A")
+class A {
   foo(x, [y]) => '$x;$y';
 }
 
-class B extends A with M1, M2, M3 native "B" {
+@Native("B")
+class B extends A with M1, M2, M3 {
 }
 
 class M1 {}
 
 class M2 {
   // These methods are only defined in this non-first, non-last mixin.
   plain(x) => 'P $x';
   bar(x, [y]) => '$y,$x';
 }
 
 class M3 {}
 
 makeB() native;
 
 void setup() native """
 function B() {}
 makeB = function(){return new B;};
 """;
 
 main() {
   setup();
 
   B b = makeB();
   Expect.equals('1;2', b.foo(1,2));
   Expect.equals('2;null', b.foo(2));
   Expect.equals('P 3', b.plain(3));
   Expect.equals('100,4', b.bar(4,100));
   Expect.equals('null,5', b.bar(5));
 }