Revert "Remove Expect from core library."

tests/standalone/io/file_write_as_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:async';
 import 'dart:io';
 import 'dart:isolate';
 
 testWriteAsBytesSync(dir) {
   var f = new File('${dir.path}/bytes_sync.txt');
   var data = [50,50,50];
   f.writeAsBytesSync(data);
   Expect.listEquals(data, f.readAsBytesSync());
   f.writeAsBytesSync(data, FileMode.APPEND);
@@ skipping 56 matching lines @@
   var tempDir = new Directory('').createTempSync();
   testWriteAsBytesSync(tempDir);
   testWriteAsStringSync(tempDir);
   testWriteAsBytes(tempDir).then((_) {
     return testWriteAsString(tempDir);
   }).then((_) {
     tempDir.deleteSync(recursive: true);
     port.close();
   });
 }