Revert "Remove Expect from core library."

tests/compiler/dart2js/type_guard_unuser_test.dart

 // Copyright (c) 2011, 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";
 import 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
 foo(a) {
   int b = foo(true);
   if (a) b = foo(2);
   return b;
 }
 """;
 
@@ skipping 41 matching lines @@
   Expect.isTrue(regexp.hasMatch(generated));
   regexp = new RegExp(getNumberTypeCheck('b'));
   Expect.isTrue(regexp.hasMatch(generated));
 
   generated = compile(TEST_THREE_WITH_BAILOUT, entry: 'foo');
   regexp = new RegExp(getNumberTypeCheck('a'));
   Expect.isTrue(regexp.hasMatch(generated));
   regexp = new RegExp(getNumberTypeCheck('b'));
   Expect.isTrue(regexp.hasMatch(generated));
 }