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

tests/compiler/dart2js_native/native_call_arity2_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";
 
 // This is a similar test to NativeCallArity1FrogTest, but makes sure
 // that subclasses also get the right number of arguments.
 
-class A native "A" {
+@Native("A")
+class A {
   int foo([x, y]) native;
 }
 
-class B extends A native "B" {
+@Native("B")
+class B extends A {
   int foo([x, y]) native;
 }
 
 makeA() native;
 makeB() native;
 
 void setup() native """
 function inherits(child, parent) {
   if (child.prototype.__proto__) {
     child.prototype.__proto__ = parent.prototype;
@@ skipping 60 matching lines @@
   Expect.equals(1, b.foo(10));
   Expect.equals(2, b.foo(null, 20));
   Expect.throws(() => b.foo(10, 20, 30));
 }
 
 main() {
   setup();
   testDynamicContext();
   testStaticContext();
 }