Fix Dart 1.17 strong-mode warnings.

test/typed_wrapper/stream_subscription_test.dart

 // Copyright (c) 2016, 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 'dart:async';
 
-import "package:async/async.dart";
 import "package:async/src/typed/stream_subscription.dart";
 import "package:test/test.dart";
 
 import '../utils.dart';
 
 void main() {
   group("with valid types, forwards", () {
     var controller;
     var wrapper;
     var isCanceled;
@@ skipping 117 matching lines @@
       });
 
       test("asFuture()", () {
         expect(wrapper.asFuture(12), completion(equals(12)));
         controller.add("foo");
         controller.close();
       });
     });
   });
 }