Revert "Remove Expect from core library."

tests/language/compile_time_constant_l_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([x = 499]) : this.x = x;
 }
 
 class B extends A {
   const B();
   final z = 99;
 }
 
 class C extends B {
   const C(this.y);
   final y;
 }
 
 const v = const C(42);
 
 main() {
   Expect.equals(42, v.y);
   Expect.equals(499, v.x);
   Expect.equals(99, v.z);
 }