Revert "Remove Expect from core library."

tests/language/compile_time_constant_i_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.redirect(x) : this(x + 1);
   const A.optional([this.x = 5]);
 }
 
 class B extends A {
   const B(x, this.y) : super(x);
   const B.redirect(x, y) : this(x + 22, y + 22);
@@ skipping 81 matching lines @@
   Expect.equals(null, c6.y);
   Expect.equals(null, c6.z);
   Expect.equals(null, c7.x);
   Expect.equals(null, c7.y);
   Expect.equals(null, c7.z);
   Expect.equals(null, c8.x);
   Expect.equals(null, c8.y);
   Expect.equals(9911, c8.z);
   Expect.identical(c3, c3b);
 }