Remove unused imports in pub.

sdk/lib/_internal/pub/test/install/hosted/install_transitive_test.dart

 // Copyright (c) 2012, 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.
 
 library pub_tests;
 
-import 'dart:io';
-
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('installs packages transitively from a pub server', () {
     servePackages([
       packageMap("foo", "1.2.3", {"bar": "2.0.4"}),
       packageMap("bar", "2.0.3"),
       packageMap("bar", "2.0.4"),
       packageMap("bar", "2.0.5")
     ]);
 
     d.appDir({"foo": "1.2.3"}).create();
 
     pubInstall();
 
     d.cacheDir({"foo": "1.2.3", "bar": "2.0.4"}).validate();
     d.packagesDir({"foo": "1.2.3", "bar": "2.0.4"}).validate();
   });
 }