Revert "Remove Expect from core library."

tests/standalone/io/file_error_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.
 //
 // Dart test program for testing error handling in file I/O.
 
-import "package:expect/expect.dart";
 import "dart:io";
 import "dart:isolate";
 
 Directory tempDir() {
   return new Directory('').createTempSync();
 }
 
 
 bool checkNonExistentFileException(e, str) {
   Expect.isTrue(e is FileIOException);
@@ skipping 446 matching lines @@
   testReadAsBytesNonExistent();
   testReadAsTextNonExistent();
   testReadAsLinesNonExistent();
   testWriteByteToReadOnlyFile();
   testWriteListToReadOnlyFile();
   testTruncateReadOnlyFile();
   testOperateOnClosedFile();
   testRepeatedlyCloseFile();
   testRepeatedlyCloseFileSync();
 }