Revert "Remove Expect from core library."

tests/language/closure_in_constructor_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";
-
 class A<T> {
   var closure;
 
   factory A.factory() => new A(() => new List<T>());
 
   A([this.closure]) {
     if (closure == null) {
       closure = () => new List<T>();
     }
   }
 }
 
 main() {
   Expect.isTrue((new A.factory()).closure() is List);
   Expect.isTrue((new A()).closure() is List);
 }