Revert "Remove Expect from core library."

tests/lib/async/stream_controller_test.dart

 // Copyright (c) 2011, 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.
 
 // Test the basic StreamController and StreamController.singleSubscription.
 library stream_controller_test;
 
-import "package:expect/expect.dart";
 import 'dart:async';
 import 'event_helper.dart';
 
 testMultiController() {
   // Test normal flow.
   var c = new StreamController.broadcast();
   Events expectedEvents = new Events()
       ..add(42)
       ..add("dibs")
       ..error("error!")
@@ skipping 386 matching lines @@
     Expect.isTrue(c.isClosed);
   }
 }
 
 main() {
   testMultiController();
   testSingleController();
   testExtraMethods();
   testClosed();
 }