Revert "Remove Expect from core library."

tests/language/try_catch3_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 try/catch statement without any exceptions
 // being thrown.
 
-import "package:expect/expect.dart";
-
 abstract class TestException {
   String getMessage();
 }
 
 class MyException implements TestException {
   const MyException([String message = ""]) : this._message = message;
   String getMessage() { return _message; }
   final String _message;
 }
 
@@ skipping 89 matching lines @@
 
 class TryCatchTest {
   static testMain() {
     Expect.equals(900, Helper.test1(1));
   }
 }
 
 main() {
   TryCatchTest.testMain();
 }