Revert "Remove Expect from core library."

tests/standalone/io/directory_non_ascii_sync_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() {
   Directory tempDir = new Directory('').createTempSync();
   var nonAsciiDir = new Directory("${tempDir.path}/æøå");
   // On MacOS you get the decomposed utf8 form of file and directory
   // names from the system. Therefore, we have to check for both here.
   var precomposed = 'æøå';
   var decomposed = new String.fromCharCodes([47, 230, 248, 97, 778]);
   Expect.isFalse(nonAsciiDir.existsSync());
   nonAsciiDir.createSync();
   Expect.isTrue(nonAsciiDir.existsSync());
   var temp = new Directory("${tempDir.path}/æøå").createTempSync();
   Expect.isTrue(temp.path.contains(precomposed) ||
                 temp.path.contains(decomposed));
   temp.deleteSync();
   tempDir.deleteSync(recursive: true);
   Expect.isFalse(nonAsciiDir.existsSync());
   Expect.isFalse(temp.existsSync());
 }