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

tests/compiler/dart2js_native/native_no_such_method_exception4_frog_test.dart

 // Copyright (c) 2012, 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:mirrors" show reflect;
+import "dart:_js_helper";
 import "package:expect/expect.dart";
 
 class GetName {
   foo(x, y) => "foo";
   baz(x, y, z) => "baz";
 }
 
 String getName(im) => reflect(new GetName()).delegate(im);;
 
-class A native "A" {
+@Native("A")
+class A {
   bar() => 42;
   noSuchMethod(x) => "native(${getName(x)}:${x.positionalArguments})";
 }
 
-class B native "B" {
+@Native("B")
+class B {
   baz() => 42;
 }
 
 makeA() native;
 
 setup() native """
   function A() {}
   makeA = function() { return new A; }
 """;
 
 main() {
   setup();
   var a = makeA();
   a.bar();
   Expect.equals("native(foo:[1, 2])", a.foo(1, 2));
   Expect.equals("native(baz:[3, 4, 5])", a.baz(3, 4, 5));
 }