pkg/scheduled_test: fix analyzer hints

test/scheduled_test/current_schedule_errors_test.dart

 // Copyright (c) 2013, 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:scheduled_test/scheduled_test.dart';
 
-import 'package:metatest/metatest.dart';
 import '../utils.dart';
 
 void main() {
   setUpMockClock();
 
   expectTestFailure('currentSchedule.errors contains the error in the '
       'onComplete queue', () {
     throw 'error';
   }, (error) => expect(error, equals('error')));
 
@@ skipping 17 matching lines @@
     mockClock.run();
 
     sleep(1).then(expectAsync((_) {
       throw 'error1';
     }));
     sleep(2).then(expectAsync((_) {
       throw 'error2';
     }));
   }, (error) => expect(error, equals('error1')));
 }