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

tests/compiler/dart2js_native/native_field_invocation4_test.dart

 // Copyright (c) 2014, 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";
 
-class A native "A" {
+@Native("A")
+class A {
   var foo;
 }
 
 class B {
   var foo;
 }
 
 nativeId(x) native;
 
 void setup() native """
 nativeId = function(x) { return x; }
 """;
 
 main() {
   setup();
   var b = new B();
   b.foo = (x) => x + 1;
   b = nativeId(b);  // Inferrer doesn't know if A has been instantiated.
   // At this point b could be A or B. The call to "foo" thus needs to go through
   // an interceptor. Tests that the interceptor doesn't screw with retrieving
   // the field and invoking the closure.
   // Use a type-check to guarantee that b is a "B".
   if (b is B) {
     Expect.equals(499, b.foo(498));
   }
 }