Make the package strong-mode clean.

test/string_accumulator_sink_test.dart

Index: test/string_accumulator_sink_test.dart
diff --git a/test/string_accumulator_sink_test.dart b/test/string_accumulator_sink_test.dart
index e6e30e6e622a105d3e9e7a164ee278ce37495099..914dd9301c014eadfa6f9de965ccaf35d83c3b79 100644
--- a/test/string_accumulator_sink_test.dart
+++ b/test/string_accumulator_sink_test.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:convert/convert.dart';
 import 'package:test/test.dart';