Revert "Remove Expect from core library."

tests/language/throw4_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 MyException1 {
   const MyException1([String message = "1"]) : message_ = message;
   final String message_;
 }
 
 class MyException2 {
   const MyException2([String message = "2"]) : message_ = message;
   final String message_;
 }
 
@@ skipping 52 matching lines @@
 
 class Throw4Test {
   static testMain() {
     Expect.equals(1900, new Helper().f1());
   }
 }
 
 main() {
   Throw4Test.testMain();
 }