Remove unused imports in pub.

sdk/lib/_internal/pub/test/install/git/require_pubspec_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('requires the dependency to have a pubspec', () {
     ensureGit();
 
     d.git('foo.git', [
       d.libDir('foo')
     ]).create();
 
     d.appDir({"foo": {"git": "../foo.git"}}).create();
 
     // TODO(nweiz): clean up this RegExp when either issue 4706 or 4707 is
     // fixed.
     pubInstall(error: 'Package "foo" doesn\'t have a pubspec.yaml file.');
   });
 }