Revert "Remove Expect from core library."

tests/standalone/io/process_working_directory_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.
 //
 // Process working directory test.
 
 library ProcessWorkingDirectoryTest;
-import "package:expect/expect.dart";
 import "dart:io";
 import "process_test_util.dart";
 
 class ProcessWorkingDirectoryTest {
   static String get fullTestFilePath {
     // Extract full path, since we run processes from another directory.
     File path = new File(getProcessTestFileName());
     Expect.isTrue(path.existsSync());
     return path.fullPathSync();
   }
@@ skipping 37 matching lines @@
     });
   }
 }
 
 
 
 main() {
   ProcessWorkingDirectoryTest.testValidDirectory();
   ProcessWorkingDirectoryTest.testInvalidDirectory();
 }