Revert "Remove Expect from core library."

tests/corelib/is_operator_basic_types_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.
 // Dart test program for the "is" type test operator.
 
-import "package:expect/expect.dart";
-
 check(args) {
   var list = args[0];
   var string = args[1];
   var nullObject = args[2];
 
   Expect.isTrue(list is Object);
   Expect.isTrue(list is List);
   Expect.isTrue(list is Collection);
   Expect.isTrue(list is Iterable);
   Expect.isFalse(list is Comparable);
@@ skipping 40 matching lines @@
 
 main() {
   // Try to make it hard for an optimizing compiler to inline the
   // tests.
   check([[], 'string', null]);
 
   // Try to make it even harder.
   var string = new String.fromCharCodes([new DateTime.now().year % 100 + 1]);
   check([string.codeUnits, string, null]);
 }