Revert "Remove Expect from core library."

tests/language/const_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.
 // Check const classes.
 
-import "package:expect/expect.dart";
-
 class AConst {
   const AConst() : b_ = 3 ;
   final int b_;
 }
 
 
 class BConst {
   const BConst();
   set foo(value) {}
   get foo { return 5; }
@@ skipping 13 matching lines @@
   var y = (const BConst())[5]++;
   Expect.equals(5, y);
 }
 
 
 main() {
   for (int i = 0; i < 1000; i++) {
     testMain();
   }
 }