Remove some unused code.

sdk/lib/_internal/pub/test/serve/watch_modified_file_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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.
 
 library pub_tests;
 
-import 'dart:io';
-
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
   initConfig();
   integration("watches modifications to files", () {
     d.dir(appPath, [
       d.appPubspec(),
       d.dir("web", [
         d.file("index.html", "before")
       ])
     ]).create();
 
     startPubServe();
     requestShouldSucceed("index.html", "before");
 
     d.dir(appPath, [
       d.dir("web", [
         d.file("index.html", "after")
       ])
     ]).create();
 
     waitForBuildSuccess();
     requestShouldSucceed("index.html", "after");
     endPubServe();
   });
 }