Remove unused imports in pub.

sdk/lib/_internal/pub/test/install/hosted/resolve_constraints_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('resolves version constraints from a pub server', () {
     servePackages([
       packageMap("foo", "1.2.3", {"baz": ">=2.0.0"}),
       packageMap("bar", "2.3.4", {"baz": "<3.0.0"}),
       packageMap("baz", "2.0.3"),
@@ skipping 11 matching lines @@
       "baz": "2.0.4"
     }).validate();
 
     d.packagesDir({
       "foo": "1.2.3",
       "bar": "2.3.4",
       "baz": "2.0.4"
     }).validate();
   });
 }