Revert "Remove Expect from core library."

tests/language/compile_time_constant_d_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, tt) : 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 12 matching lines @@
   Expect.equals("A 3 499 99 100", a1.toString());
 
   Expect.isTrue(a1 === a2);
 
   Expect.equals(1, a3.x);
   Expect.equals(2, a3.y);
   Expect.equals(3, a3.z);
   Expect.equals(4, a3.t);
   Expect.equals("A 1 2 3 4", a3.toString());
 }