Revert "Remove Expect from core library."

tests/language/getters_setters_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 GettersSettersTest {
 
   static int foo;
 
   static testMain() {
     A a = new A();
     a.x = 2;
     Expect.equals(2, a.x);
     Expect.equals(2, a.x_);
 
@@ skipping 125 matching lines @@
   }
 
   int getIt() {
      return super.x_;
   }
 }
 
 main() {
   GettersSettersTest.testMain();
 }