Revert "Remove Expect from core library."

tests/lib/async/stream_event_transform_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.
 
 library stream_event_transform_test;
 
-import "package:expect/expect.dart";
 import 'dart:async';
 import '../../../pkg/unittest/lib/unittest.dart';
 import 'event_helper.dart';
 
 void handleData(int data, EventSink<int> sink) {
   sink.addError(new AsyncError("$data"));
   sink.add(data + 1);
 }
 
 void handleError(AsyncError e, EventSink<int> sink) {
@@ skipping 55 matching lines @@
             handleData: handleData,
             handleError: handleError,
             handleDone: handleDone
         )));
     actual.onDone(() {
       Expect.listEquals(expected.events, actual.events);
     });
     input.replay(c);
   }
 }