Get rid of an unused library.

test/frontend/set_up_all_test.dart

 // Copyright (c) 2015, 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:async/async.dart';
 import 'package:test/test.dart';
 
 import '../utils.dart';
 
 void main() {
   test("runs once before all tests", () {
     return expectTestsPass(() {
       var setUpAllRun = false;
       setUpAll(() {
@@ skipping 298 matching lines @@
         setUpAll(() => throw "error");
         setUpAll(shouldNotRun);
 
         test("test", shouldNotRun);
       });
 
       expect(await engine.run(), isFalse);
     });
   });
 }