Revert "Remove Expect from core library."

tests/language/default_factory_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";
-
 // Dart test program for testing default factories.
 
 abstract class Vehicle {
   factory Vehicle() = GoogleOne.Vehicle;
 }
 
 
 class Bike implements Vehicle, GoogleOne {
   Bike.redOne() {}
 }
 
 
 abstract class SpaceShip {
   factory SpaceShip() = GoogleOne;
 }
 
 
 class GoogleOne implements SpaceShip {
   GoogleOne.internal_() {}
   factory GoogleOne() { return new GoogleOne.internal_(); }
   factory GoogleOne.Vehicle() { return new Bike.redOne(); }
 }
 
 
 main() {
   Expect.equals(true, (new Bike.redOne()) is Bike);
   Expect.equals(true, (new SpaceShip()) is GoogleOne);
   Expect.equals(true, (new Vehicle()) is Bike);
 }