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

tests/compiler/dart2js_native/downcast_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.
 
 // Test for downcasts on native classes.
 
+import "dart:_js_helper";
 import "package:expect/expect.dart";
 
 abstract class J {
 }
 
 abstract class I extends J {
   I read();
   write(I x);
 }
 
 // Native implementation.
 
-class A implements I native "A" {
+@Native("A")
+class A implements I {
   // The native class accepts only other native instances.
   A read() native;
   write(A x) native;
 }
 
-class B extends A native "B" {
+@Native("B")
+class B extends A {
 }
 
 makeA() native;
 makeB() native;
 
 void setup() native """
 // This code is all inside 'setup' and so not accesible from the global scope.
 function inherits(child, parent) {
   if (child.prototype.__proto__) {
     child.prototype.__proto__ = parent.prototype;
@@ skipping 35 matching lines @@
   Expect.equals(b1, b1 as I);
   Expect.equals(b1, b1 as A);
   Expect.equals(b1, b1 as B);
 
   Expect.equals(a1, a1 as J);
   Expect.equals(a1, a1 as I);
   Expect.equals(a1, a1 as A);
   Expect.throws(() => a1 as B);
   Expect.throws(() => a1 as C);
 }