Revert "Remove Expect from core library."

tests/standalone/io/file_constructor_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';
 
 void main() {
   bool developerMode = false;
   assert(developerMode = true);
   new File('blåbærgrød');
   new File('foo.txt');
   try {
     new File(null);
     Expect.fail('ArgumentError expected.');
   } on ArgumentError catch(e) {
     // Expected.
   }
   try {
     new File(1);
     Expect.fail('Error expected.');
   } on ArgumentError catch(e) {
     if (developerMode) throw;
   } on TypeError catch(e) {
     if (!developerMode) throw;
   }
 }