Remove unused async import. (dart-lang/stream_transform#113)

Since this version does not support dart 2.0, the async import is
unnecessary.

pkgs/stream_transform/lib/src/tap.dart

diff --git a/pkgs/stream_transform/lib/src/tap.dart b/pkgs/stream_transform/lib/src/tap.dart
index 1942b12..2696e02 100644
--- a/pkgs/stream_transform/lib/src/tap.dart
+++ b/pkgs/stream_transform/lib/src/tap.dart
@@ -1,8 +1,6 @@
 // Copyright (c) 2017, 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 'from_handlers.dart';
 
 /// A utility to chain extra behavior on a stream.