Revert "Remove Expect from core library."

tests/language/closure_type_variables_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 type variables are available in closures.
 
 class A<T> {
   A();
 
   A.bar() {
     g() {
       new A<T>();
     }
     g();
   }
 
   foo() {
     g() {
       return new A<T>();
     }
     return g();
   }
 }
 
 main() {
   Expect.isTrue(new A<int>().foo() is A<int>);
   Expect.isTrue(new A<int>.bar().foo() is A<int>);
 }