Make the package strong-mode clean.

test/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 AccumulatorSink<int>();
   });
 
   test("provides access to events as they're added", () {
@@ skipping 24 matching lines @@
     expect(sink.isClosed, isFalse);
     sink.close();
     expect(sink.isClosed, isTrue);
   });
 
   test("doesn't allow add() to be called after close()", () {
     sink.close();
     expect(() => sink.add(1), throwsStateError);
   });
 }