Fix Dart 1.17 strong-mode warnings.

test/restartable_timer_test.dart

 // Copyright (c) 2015, 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:async/async.dart';
 import 'package:fake_async/fake_async.dart';
 import 'package:test/test.dart';
 
 main() {
   test("runs the callback once the duration has elapsed", () {
     new FakeAsync().run((async) {
       var fired = false;
-      var timer = new RestartableTimer(new Duration(seconds: 5), () {
+      new RestartableTimer(new Duration(seconds: 5), () {
         fired = true;
       });
 
       async.elapse(new Duration(seconds: 4));
       expect(fired, isFalse);
 
       async.elapse(new Duration(seconds: 1));
       expect(fired, isTrue);
     });
   });
@@ skipping 68 matching lines @@
       expect(fired, isFalse);
       timer.reset();
 
       async.elapse(new Duration(seconds: 5));
       expect(fired, isTrue);
     });
   });
 
   test("only runs the callback once if the timer isn't reset", () {
     new FakeAsync().run((async) {
-      var timer = new RestartableTimer(
+      new RestartableTimer(
           new Duration(seconds: 5),
           expectAsync(() {}, count: 1));
       async.elapse(new Duration(seconds: 10));
     });
   });
 }