Make Zone API strong mode clean.

tests/lib/async/zone_run_guarded_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() {
+  var result;
+
   Completer done = new Completer();
   List events = [];
 
   // runGuarded calls run, captures the synchronous error (if any) and
   // gives that one to handleUncaughtError.
 
   Expect.identical(Zone.ROOT, Zone.current);
   Zone forked;
   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");
     return parent.run(origin, f);
   }, handleUncaughtError:
               (Zone self, ZoneDelegate parent, Zone origin, error, stackTrace) {
     Expect.identical(Zone.ROOT, Zone.current);
     Expect.identical(forked, origin);
     events.add("forked.handleUncaught $error");
-    return 499;
+    result = 499;
   }));
 
-  var result = forked.runGuarded(() {
+  forked.runGuarded(() {
     events.add("runGuarded 1");
     Expect.identical(forked, Zone.current);
-    return 42;
+    result = 42;
   });
   Expect.identical(Zone.ROOT, Zone.current);
   Expect.equals(42, result);
   events.add("after runGuarded 1");
 
-  result = forked.runGuarded(() {
+  forked.runGuarded(() {
     events.add("runGuarded 2");
     Expect.identical(forked, Zone.current);
     throw 42;
   });
   Expect.equals(499, result);
 
   Expect.listEquals([
     "forked.run",
     "runGuarded 1",
     "after runGuarded 1",
     "forked.run",
     "runGuarded 2",
     "forked.handleUncaught 42"
   ], events);
 
+  result = null;
   events.clear();
   asyncStart();
-  result = forked.runGuarded(() {
+  forked.runGuarded(() {
     Expect.identical(forked, Zone.current);
     events.add("run closure");
     forked.scheduleMicrotask(() {
       events.add("run closure 2");
       Expect.identical(forked, Zone.current);
       done.complete(true);
       throw 88;
     });
     throw 1234;
   });
   events.add("after nested scheduleMicrotask");
   Expect.equals(499, result);
 
   done.future.whenComplete(() {
     Expect.listEquals([
       "forked.run",
       "run closure",
       "forked.handleUncaught 1234",
       "after nested scheduleMicrotask",
       "forked.run",
       "run closure 2",
       "forked.handleUncaught 88"
     ], events);
     asyncEnd();
   });
 }