Revert "Remove Expect from core library."

tests/language/compile_time_constant_e_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;
   final y;
   final z;
   final t;
 
   const A([this.z = 99, tt = 100]) : y = 499, t = tt, x = 3;
   const A.n({this.z: 99, tt: 100}) : y = 499, t = tt, x = 3;
   const A.named({z, this.t}) : y = 400 + z, this.z = z, x = 3;
   const A.named2({t, z, y, x}) : x = t, y = z, z = y, t = x;
@@ skipping 50 matching lines @@
   Expect.equals("A null null null null", a8.toString());
 
   Expect.isTrue(a3 === a9);
 
   Expect.equals(4, a10.x);
   Expect.equals(3, a10.y);
   Expect.equals(2, a10.z);
   Expect.equals(1, a10.t);
   Expect.equals("A 4 3 2 1", a10.toString());
 }