Revert "Remove Expect from core library."

tests/language/context_args_with_defaults_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 ContextArgsWithDefaultsTest {
   static void testMain() {
     crasher(1, 'foo')();
   }
 
   static crasher(int fixed, [String optional = '']) {
     return () {
       Expect.equals(1, fixed);
       Expect.equals('foo', optional);
     };
   }
 
 }
 
 main() {
   ContextArgsWithDefaultsTest.testMain();
 }