Revert "Remove Expect from core library."

tests/language/compile_time_constant_f_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 {
   final x;
   const A(this.x);
   const A.named([this.x]);
   const A.named2([this.x = 2]);
 }
 
 const a1 = const A(0);
 const a2 = const A.named();
 const a3 = const A.named(1);
 const a4 = const A.named2();
 const a5 = const A.named2(3);
 
 main() {
   Expect.equals(0, a1.x);
   Expect.equals(null, a2.x);
   Expect.equals(1, a3.x);
   Expect.equals(2, a4.x);
   Expect.equals(3, a5.x);
 }