Revert "Remove Expect from core library."

tests/lib/async/future_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.
 
 library future_test;
 
-import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:isolate';
 
 const Duration MS = const Duration(milliseconds: 1);
 
 testImmediate() {
   final future = new Future<String>.immediate("42");
   var port = new ReceivePort();
   future.then((x) {
     Expect.equals("42", x);
@@ skipping 625 matching lines @@
   testFutureCatchThrowsAsync();
   testFutureWhenThrowsAsync();
   testFutureCatchRethrowsAsync();
 
   testChainedFutureValue();
   testChainedFutureValueDelay();
   testChainedFutureError();
 }