pkg/unittest: fix imports

test/runner/browser/chrome_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 'dart:io';
 
 import 'package:unittest/unittest.dart';
 import 'package:unittest/src/runner/browser/chrome.dart';
 import 'package:unittest/src/util/io.dart';
 import 'package:shelf/shelf.dart' as shelf;
 import 'package:shelf/shelf_io.dart' as shelf_io;
 import 'package:shelf_web_socket/shelf_web_socket.dart';
 
 void main() {
   group("running JavaScript", () {
@@ skipping 106 matching lines @@
       return chrome.close().whenComplete(server.close);
     });
   });
 
   test("reports an error in onExit", () {
     var chrome = new Chrome("http://dart-lang.org",
         executable: "_does_not_exist");
     expect(chrome.onExit, throwsA(new isInstanceOf<ProcessException>()));
   });
 }