Remove unused dart:async imports.

As of Dart 2.1, Future/Stream have been exported from dart:core.

pkgs/watcher/test/file_watcher/shared.dart

diff --git a/pkgs/watcher/test/file_watcher/shared.dart b/pkgs/watcher/test/file_watcher/shared.dart
index c837a21..9b72f0e 100644
--- a/pkgs/watcher/test/file_watcher/shared.dart
+++ b/pkgs/watcher/test/file_watcher/shared.dart
@@ -2,8 +2,6 @@
 // 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:test/test.dart';
 
 import '../utils.dart';

pkgs/watcher/test/utils.dart

diff --git a/pkgs/watcher/test/utils.dart b/pkgs/watcher/test/utils.dart
index 06d1a12..6e7686c 100644
--- a/pkgs/watcher/test/utils.dart
+++ b/pkgs/watcher/test/utils.dart
@@ -2,7 +2,6 @@
 // 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 'dart:io';
 
 import 'package:async/async.dart';