Revert "Remove Expect from core library."

tests/language/rethrow_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 program for testing throw statement
 
-import "package:expect/expect.dart";
-
 class MyException {
   const MyException();
 }
 
 class OtherException {
   const OtherException();
 }
 
 class RethrowTest {
   MyException currentException;
@@ skipping 36 matching lines @@
       Expect.equals(true, identical(e, currentException));
     }
   }
 }
 
 main() {
   RethrowTest t = new RethrowTest();
   t.testRethrow();
   t.testRethrowPastUncaught();
 }