Revert "Remove Expect from core library."

tests/language/argument_definition3_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.
 
-import "package:expect/expect.dart";
-
 main() {
   new Param.test1(false);
   new Param.test1(true, 42);
 
   new Param.test2(false);
   new Param.test2(true, 42);
 
   new Param.test3(false);
   new Param.test3(true, 42);
 
@@ skipping 52 matching lines @@
 
   Param.test6(passed, [int a = 0]) : field = ?a, otherField = a++ {
     Expect.equals(passed, ?a);
     Expect.equals(passed, field);
     Expect.equals(passed ? 43 : 1, a);
   }
 
   Param.test7() : super.withUpdate(true, 42);
   Param.test8() : super.withUpdate(false);
 }