Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/phi_gvn_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";
-
 foo(x) { }
 
 main() {
   for (var i = 0; i < 1; i++) {
     foo(i + 1);
     Expect.equals(0, i);
   }
 }