Revert "Remove Expect from core library."

tests/standalone/io/windows_environment_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.
 
-import "package:expect/expect.dart";
 import "dart:io";
 
 main() {
   if (Platform.operatingSystem != 'windows') return;
   var tempDir = new Directory('').createTempSync();
   var funkyDir = new Directory("${tempDir.path}/å");
   funkyDir.createSync();
   var funkyFile = new File('${funkyDir.path}/funky.bat');
   funkyFile.writeAsStringSync("""
 @echo off
 set SCRIPTDIR=%~dp0
 %1 %2
       """);
   var options = new Options();
   var dart = options.executable;
   var scriptDir = new Path(options.script).directoryPath;
   var script = scriptDir.append('windows_environment_script.dart');
   Process.run('cmd',
               ['/c', funkyFile.path, dart, script.toNativePath()]).then((p) {
     Expect.equals(0, p.exitCode);
     tempDir.deleteSync(recursive: true);
   });
 }