Revert "Remove Expect from core library."

tests/language/lazy_static4_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";
-
 final x = foo(499);
 final y = foo(41) + 1;
 
 final t = bar(499);
 final u = bar(41) + 1;
 final v = bar("some string");
 
 foo(x) => x; // The return type will always be int.
 bar(x) => x; // The return type varies and can be int or String.
 
 main() {
   Expect.equals(499, x);
   Expect.equals(42, y);
   Expect.equals(499, t);
   Expect.equals(42, u);
   Expect.equals("some string", v);
 }