Make the package strong-mode clean.

test/string_accumulator_sink_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:convert/convert.dart';
 import 'package:test/test.dart';
 
 void main() {
   var sink;
   setUp(() {
     sink = new StringAccumulatorSink();
   });
 
   test("provides access to the concatenated string", () {
@@ skipping 32 matching lines @@
   test("doesn't allow add() to be called after close()", () {
     sink.close();
     expect(() => sink.add("x"), throwsStateError);
   });
 
   test("doesn't allow addSlice() to be called after close()", () {
     sink.close();
     expect(() => sink.addSlice("", 0, 0, false), throwsStateError);
   });
 }