Revert "Remove Expect from core library."

tests/language/first_class_types_constants_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 C<T> {
   final T t;
   const C(this.t);
 }
 
 typedef int Fun(bool, String);
 
 const c0 = C;
 const c1 = const C(C);
 const c2 = Fun;
 const c3 = const C(Fun);
 
 main() {
   Expect.identical(C, C);
   Expect.identical(C, c0);
   Expect.identical(c1, c1);
   Expect.notEquals(c0, c1);
   Expect.notEquals(c1, c2);
   Expect.identical(c1.t, c0);
   Expect.notEquals(C, Fun);
   Expect.identical(Fun, Fun);
   Expect.identical(Fun, c2);
   Expect.identical(c3.t, c2);
 }