Revert "Remove Expect from core library."

tests/isolate/isolate_negative_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.
 
 // Dart test program for testing that isolates are spawned.
 
 library IsolateNegativeTest;
-import "package:expect/expect.dart";
 import 'dart:isolate';
 import '../../pkg/unittest/lib/unittest.dart';
 
 void entry() {
   port.receive((ignored, replyTo) {
     replyTo.send("foo", null);
   });
 }
 
 main() {
   test("ensure isolate code is executed", () {
     SendPort port = spawnFunction(entry);
     port.call("foo").then(expectAsync1((message) {
       expect("Expected fail", isTrue);   // <=-------- Should fail here.
     }));
   });
 }