Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/for_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";
-
 void for1() {
   var cond = true;
   var result = 0;
   for (var x = 0; cond; x = x + 1) {
     if (x == 10) cond = false;
     result = result + x;
   }
   Expect.equals(55, result);
 }
 
@@ skipping 63 matching lines @@
 }
 
 void main() {
   for1();
   for2();
   for3();
   for4();
   for5();
   for6();
 }