Revert "Remove Expect from core library."

tests/language/metadata_syntax_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";
-
 // Test that is is an error to use type arguments in metadata without
 // parens.
 
 @Bar()
 @List<String> /// 01: compile-time error
 class Bar {
   @Bar()
   @List<String> /// 02: compile-time error
   const Bar();
 
   @Bar()
   @List<String> /// 03: compile-time error
   final x = '';
 }
 
 main() {
   Expect.equals('', new Bar().x);
   Expect.equals('', const Bar().x);
 }