Revert "Remove Expect from core library."

tests/language/list_literal_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";
-
 abstract class I {}
 
 void main() {
   var list;
   list = <int
     I /// 00: compile-time error
     , int /// 01: compile-time error
     >[0];
   Expect.equals(1, list.length);
 
@@ skipping 31 matching lines @@
   Expect.equals(1, list.length);
 
   list = <int>[<int>[<int>[1][0]][0]];
   Expect.equals(1, list.length);
   Expect.equals(1, list[0]);
 
   list = <int>[<List<int>>[[1]][0][0]];
   Expect.equals(1, list.length);
   Expect.equals(1, list[0]);
 }