Revert "Remove Expect from core library."

tests/html/window_nosuchmethod_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.
 
 library WindowNSMETest;
-import "package:expect/expect.dart";
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html' as dom;
 
 // Not defined in dom.Window.
 foo(x) => x;
 
 class Unused {
   foo(x) => 'not $x';
 }
@@ skipping 29 matching lines @@
       try {
         String x = dom.window.foo(message);
         expect(false, isTrue, reason: 'Should not reach here: $x');
       } on NoSuchMethodError catch (e) {
         // Expected exception.
       } on Exception catch (e) {
         expect(false, isTrue, reason: 'Wrong exception: $e');
       }
     });
 }