Make Zone API strong mode clean.

tests/lib_strong/async/zone_run_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();
   List events = [];
 
   bool shouldForward = true;
   Expect.identical(Zone.ROOT, Zone.current);
   Zone forked = Zone.current.fork(specification: new ZoneSpecification(
-      run: (Zone self, ZoneDelegate parent, Zone origin, f()) {
+      run: <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.ROOT, Zone.current);
     events.add("forked.run");
     if (shouldForward) return parent.run(origin, f);
-    return 42;
+    return 42 as R;
   }));
 
   events.add("zone forked");
   Zone expectedZone = forked;
   var result = forked.run(() {
     Expect.identical(expectedZone, Zone.current);
     events.add("run closure");
     return 499;
   });
   Expect.equals(499, result);
@@ skipping 52 matching lines @@
       });
     });
     zone3.run(() {
       future.then((_) {
         Expect.identical(zone3, Zone.current);
         asyncEnd();
       });
     });
   });
 }