Revert "Remove Expect from core library."

tests/language/super_setter_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.
 // Dart test for testing super setters and getters.
 
-import "package:expect/expect.dart";
-
 class Base {
   Base() {}
   String value_;
 
   String get value { return value_; }
   String set value(String newValue) {
     value_ = 'Base:$newValue';
   }
 }
 
@@ skipping 12 matching lines @@
   static void testMain() {
     final b = new Derived();
     b.value = "foo";
     Expect.equals("Base:Derived:foo", b.value);
   }
 }
 
 main() {
   SuperSetterTest.testMain();
 }