Revert "Remove Expect from core library."

tests/language/compile_time_constant_k_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";
-
 const x = const { 'a': 3, 'a': 4 };
 const y = const { 'a': 10, 'b': 11, 'a': 12, 'b': 13, 'a': 14 };
 const z = const { '__proto__': 496,
                   '__proto__': 497,
                   '__proto__': 498,
                   '__proto__': 499 };
 
 const x2 = const { 'a': 4 };
 const y2 = const { 'a': 14, 'b': 13 };
 const z2 = const { '__proto__': 499 };
 
 main() {
   Expect.identical(x2, x);
   Expect.identical(y2, y);
   Expect.identical(z2, z);
 }