Revert "Remove Expect from core library."

tests/language/throw1_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 throw statement
 
-import "package:expect/expect.dart";
-
 abstract class TestException {
   String getMessage();
 }
 
 class MyException implements TestException {
   const MyException([String message = ""]) : message_ = message;
   String getMessage() { return message_; }
   final String message_;
 }
 
@@ skipping 46 matching lines @@
 
 class Throw1Test {
   static testMain() {
     Expect.equals(850, Helper.f1(1));
   }
 }
 
 main() {
   Throw1Test.testMain();
 }