Revert "Remove Expect from core library."

tests/language/try_catch_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.
 
-import "package:expect/expect.dart";
-
 class MyException { }
 
 class MyException1 extends MyException { }
 
 class MyException2 extends MyException { }
 
 class TryCatchTest {
   static void test1() {
     var foo = 0;
     try {
@@ skipping 129 matching lines @@
     test6();
     test7();
     test8();
     test9();
   }
 }
 
 main() {
   TryCatchTest.testMain();
 }