Revert "Remove Expect from core library."

tests/language/instanceof4_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 testing the instanceof operation.
 // Regression test for issue 5216.
 
-import "package:expect/expect.dart";
-
 class Foo<T> {
   bool isT() => "a string" is T;
   bool isNotT() => "a string" is! T;
   bool isListT() => [0, 1, 2] is List<T>;
   bool isNotListT() => [0, 1, 2] is! List<T>;
   bool isAlsoListT() => <int>[0, 1, 2] is List<T>;
   bool isNeitherListT() => <int>[0, 1, 2] is! List<T>;
 }
 
 testFooString() {
@@ skipping 43 matching lines @@
   Expect.isTrue(o.isListT());
   Expect.isTrue(!o.isNotListT());
   Expect.isTrue(o.isAlsoListT());
   Expect.isTrue(!o.isNeitherListT());
 }
 
 main() {
   testFooString();
   testFooInt();
 }