Remove unused imports in pub.

sdk/lib/_internal/pub/test/hosted/remove_removed_dependency_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();
 
   forBothPubInstallAndUpdate((command) {
     integration("removes a dependency that's removed from the pubspec", () {
       servePackages([
         packageMap("foo", "1.0.0"),
@@ skipping 13 matching lines @@
 
       pubCommand(command);
 
       d.packagesDir({
         "foo": "1.0.0",
         "bar": null
       }).validate();
     });
   });
 }