Make Zone API strong mode clean.

tests/lib_strong/async/zone_run_unary_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(
-      runUnary: (Zone self, ZoneDelegate parent, Zone origin, f(arg), arg) {
+  Zone forked = Zone.current.fork(specification: new ZoneSpecification(runUnary:
+      <R, T>(Zone self, ZoneDelegate parent, Zone origin, R f(arg), T arg) {
     // 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);
     events.add("forked.run1");
     if (shouldForward) return parent.runUnary(origin, f, arg + 1);
-    return 42;
+    return 42 as R;
   }));
 
   events.add("zone forked");
   Zone expectedZone = forked;
   var result = forked.runUnary((arg) {
     Expect.identical(expectedZone, Zone.current);
     events.add("run closure");
     return arg + 3;
   }, 495);
   Expect.equals(499, result);
@@ skipping 30 matching lines @@
       "forked.run1",
       "executed run2",
       "forked.run1",
       "run closure 2",
       "after nested scheduleMicrotask",
       "run closure 3"
     ], events);
     asyncEnd();
   });
 }