Revert "Remove Expect from core library."

tests/language/throw6_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 Helper {
   Helper() : i = 0 { }
 
   int f1() {
     int j = 0;
@@ skipping 30 matching lines @@
 
 class Throw6Test {
   static testMain() {
     Expect.equals(1900, new Helper().f1());
   }
 }
 
 main() {
   Throw6Test.testMain();
 }