Revert "Remove Expect from core library."

tests/language/list_is_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";
-
 class A<T> {
   bar() => new List<T>();
 }
 
 main() {
   check(new List(), true, true, true);
   check(new List<int>(), true, true, false);
   check(new A().bar(), true, true, true);
   check(new A<double>().bar(), true, false, true);
   check(new Object(), false, false, false);
 }
 
 check(val, expectList, expectListInt, expectListDouble) {
   Expect.equals(expectList, val is List);
   Expect.equals(expectListInt, val is List<int>);
   Expect.equals(expectListDouble, val is List<double>);
 }