Revert "Remove Expect from core library."

tests/language/core_type_check_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";
-
 check(value, expectComparable, expectPattern) {
   Expect.equals(expectComparable, value is Comparable);
   Expect.equals(expectPattern, value is Pattern);
 }
 
 int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
 
 class A implements Comparable {
 }
 
@@ skipping 13 matching lines @@
   check(things[inscrutable(0)], false, false); // List
   check(things[inscrutable(1)], true, false); // int
   check(things[inscrutable(2)], true, false); // num
   check(things[inscrutable(3)], true, true); // string
   check(things[inscrutable(4)], false, false); // Object
   check(things[inscrutable(5)], true, false); // A
   check(things[inscrutable(6)], false, false); // B
   check(things[inscrutable(7)], false, true); // C
   check(things[inscrutable(8)], true, true); // D
 }