Revert "Remove Expect from core library."

tests/standalone/pow_test.dart

 // Copyright (c) 2013, 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.
 //
 // Dart test program for testing math's pow.
 
 library pow_test;
 
-import "package:expect/expect.dart";
 import 'dart:math';
 
 var expectedResults =
 [
   1,
   2,
   4,
   8,
   16,
   32,
@@ skipping 69 matching lines @@
   }
   // Optimize it.
   for (int i = 0; i < 8888; i++) {
     pow(2, 3);
   }
   exp = 0;
   for (int val in expectedResults) {
     Expect.equals(val, pow(2, exp++));
   }
 }