Revert "Remove Expect from core library."

tests/language/argument_definition5_test.dart

 // Copyright (c) 2013, 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";
-
 var x = "";
 
 test(String str, [onError(String)]) {
   return (() {
     try {
       throw "";
     } catch(e) {
       if (?onError) {
         onError(str);
       } else {
         x = "${str} error";
       }
     }
   });
 }
 
 ignoreError(String str) {
   x = "${str} error ignored";
 }
 
 main() {
   Expect.equals("", x);
   test("test")();
   Expect.equals("test error", x);
   test("test", ignoreError)();
   Expect.equals("test error ignored", x);
 }