Revert "Remove Expect from core library."

tests/language/factory_redirection2_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";
-
 // Test that it is a compile-time error to have a redirection in a
 // non-factory constructor.
 
 class Foo {
   Foo()
   = Bar /// 01: compile-time error
   ;
 }
 
 class Bar extends Foo {
   factory Bar() => null;
 }
 
 main() {
   Expect.isTrue(new Foo() is Foo);
   Expect.isFalse(new Foo() is Bar);
 }