Make Zone API strong mode clean.

tests/lib_strong/async/zone_register_callback_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 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'dart:async';
 
 main() {
   Completer done = new Completer();
 
   var valueToCapture;
   var restoredValue;
 
   Expect.identical(Zone.ROOT, Zone.current);
   Zone forked = Zone.current.fork(specification: new ZoneSpecification(
-      registerCallback: (Zone self, ZoneDelegate parent, Zone origin, f()) {
+      registerCallback:
+          <R>(Zone self, ZoneDelegate parent, Zone origin, R f()) {
     // The zone is still the same as when origin.run was invoked, which
     // is the root zone. (The origin zone hasn't been set yet).
     Expect.identical(Zone.current, Zone.ROOT);
     // Note that not forwarding is completely legal, though not encouraged.
     var capturedValue = valueToCapture;
     return parent.registerCallback(origin, () {
       restoredValue = capturedValue;
       return f();
     });
   }));
 
   valueToCapture = 499;
   var fun = () => 99;
   var registered = forked.registerCallback(fun);
   Expect.isFalse(identical(fun, registered));
 
   // It is legal to invoke the callback in a different zone. This is, of course,
   // extremely discouraged.
   var result = registered();
   Expect.equals(99, result);
   Expect.equals(499, restoredValue);
 }